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‎EvaluateReversePolishNotation/evaluate_reverse_polish_notation.dart

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/*
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-* 150. Evaluate Reverse Polish Notation *-
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Evaluate the value of an arithmetic expression in Reverse Polish Notation.
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Valid operators are +, -, *, and /. Each operand may be an integer or another expression.
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Note that division between two integers should truncate toward zero.
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It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.
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Example 1:
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Input: tokens = ["2","1","+","3","*"]
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Output: 9
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Explanation: ((2 + 1) * 3) = 9
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Example 2:
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Input: tokens = ["4","13","5","/","+"]
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Output: 6
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Explanation: (4 + (13 / 5)) = 6
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Example 3:
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Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
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Output: 22
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Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
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= ((10 * (6 / (12 * -11))) + 17) + 5
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= ((10 * (6 / -132)) + 17) + 5
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= ((10 * 0) + 17) + 5
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= (0 + 17) + 5
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= 17 + 5
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= 22
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Constraints:
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1 <= tokens.length <= 104
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tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].
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*/
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import 'dart:collection';
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class A {
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int evalRPN(List<String> tokens) {
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late int a, b;
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List<int> s = [];
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for (String token in tokens) {
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try {
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s.add(int.parse(token));
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} catch (e) {
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a = s.removeLast();
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b = s.removeLast();
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switch (token) {
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case "+":
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s.add(b + a);
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break;
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case "-":
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s.add(b - a);
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break;
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case "*":
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s.add(b * a);
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break;
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case "/":
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s.add(b ~/ a);
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break;
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}
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}
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}
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return s.removeLast();
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}
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}
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class B {
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int evalRPN(List<String> tokens) {
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if (tokens.isEmpty || tokens.length == 0) {
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return 0;
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}
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int len = tokens.length;
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List<int> result = List.filled(len, 0);
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int pointer = 0;
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for (int i = 0; i < len; i++) {
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String current = tokens[i];
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if (isDigit(current[current.length - 1])) {
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result[pointer++] = int.parse(current);
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} else {
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if (current == "+") {
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result[pointer - 2] += result[--pointer];
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} else if (current == "-") {
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result[pointer - 2] -= result[--pointer];
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} else if (current == "*") {
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result[pointer - 2] *= result[--pointer];
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} else {
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result[pointer - 2] ~/= result[--pointer];
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}
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}
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}
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return result[0];
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}
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bool isDigit(String s) {
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if (s.isEmpty) {
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return false;
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}
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return int.tryParse(s) != null;
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}
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}
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class C {
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int evalRPN(List<String> tokens) {
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List<int> s = [];
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HashSet<String> op = HashSet()..addAll(["+", "-", "*", "/"]);
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for (String c in tokens) {
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if (op.contains(c)) {
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int p2 = s.removeLast();
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int p1 = s.removeLast();
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s.add(execOp(p1, p2, c));
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} else {
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s.add(int.parse(c));
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}
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}
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return s.removeLast();
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}
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int execOp(int p1, int p2, String op) {
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switch (op) {
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case "+":
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return p1 + p2;
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case "-":
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return p1 - p2;
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case "*":
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return p1 * p2;
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case "/":
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return p1 ~/ p2;
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}
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return 0;
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}
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}

‎EvaluateReversePolishNotation/evaluate_reverse_polish_notation.go

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package main
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import "strconv"
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var m = map[string]func(int, int) int{
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"+": func(lhs, rhs int) int { return lhs + rhs },
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"-": func(lhs, rhs int) int { return lhs - rhs },
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"*": func(lhs, rhs int) int { return lhs * rhs },
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"/": func(lhs, rhs int) int { return lhs / rhs },
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}
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func evalRPN(tokens []string) int {
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var stk []int
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for _, val := range tokens {
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var num int
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if fn, ok := m[val]; ok {
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var lhs, rhs int
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stk, lhs, rhs = popTopTwo(stk)
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num = fn(lhs, rhs)
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} else {
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num, _ = strconv.Atoi(val)
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}
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stk = append(stk, num)
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}
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return stk[0]
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}
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func popTopTwo(stk []int) ([]int, int, int) {
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rhs := stk[len(stk)-1]
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stk = stk[:len(stk)-1]
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lhs := stk[len(stk)-1]
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stk = stk[:len(stk)-1]
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return stk, lhs, rhs
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}

‎EvaluateReversePolishNotation/evaluate_reverse_polish_notation.md

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# 🔥 3 Approaches 🔥 || Simple Fast and Easy || with Explanation
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## Explanation
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Loop through tokens, push integers into a stack, and pop two from stack if token is not an integer, calculate the result, push result back into stack.
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## Solution - 1 STACK
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```dart
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class Solution {
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int evalRPN(List<String> tokens) {
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late int a, b;
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List<int> s = [];
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for (String token in tokens) {
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try {
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s.add(int.parse(token));
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} catch (e) {
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a = s.removeLast();
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b = s.removeLast();
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switch (token) {
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case "+":
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s.add(b + a);
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break;
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case "-":
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s.add(b - a);
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break;
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case "*":
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s.add(b * a);
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break;
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case "/":
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s.add(b ~/ a);
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break;
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}
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}
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}
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return s.removeLast();
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}
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}
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```
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## Solution - 2
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```dart
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bool isDigit(String s) {
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if (s.isEmpty) {
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return false;
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}
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return int.tryParse(s) != null;
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}
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}
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```
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```dart
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class Solution {
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int evalRPN(List<String> tokens) {
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if (tokens.isEmpty || tokens.length == 0) {
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return 0;
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}
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int len = tokens.length;
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List<int> result = List.filled(len, 0);
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int pointer = 0;
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for (int i = 0; i < len; i++) {
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String current = tokens[i];
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if (isDigit(current[current.length - 1])) {
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result[pointer++] = int.parse(current);
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} else {
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if (current == "+") {
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result[pointer - 2] += result[--pointer];
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} else if (current == "-") {
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result[pointer - 2] -= result[--pointer];
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} else if (current == "*") {
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result[pointer - 2] *= result[--pointer];
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} else {
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result[pointer - 2] ~/= result[--pointer];
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}
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}
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}
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return result[0];
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}
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```
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## Solution - 3
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```dart
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class Solution {
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int evalRPN(List<String> tokens) {
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List<int> s = [];
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HashSet<String> op = HashSet()..addAll(["+", "-", "*", "/"]);
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for (String c in tokens) {
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if (op.contains(c)) {
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int p2 = s.removeLast();
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int p1 = s.removeLast();
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s.add(execOp(p1, p2, c));
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} else {
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s.add(int.parse(c));
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}
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}
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return s.removeLast();
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}
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int execOp(int p1, int p2, String op) {
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switch (op) {
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case "+":
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return p1 + p2;
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case "-":
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return p1 - p2;
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case "*":
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return p1 * p2;
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case "/":
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return p1 ~/ p2;
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}
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return 0;
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}
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}
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```

‎README.md

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- [**931.** Minimum Falling Path Sum](MinimumFallingPathSum/minimum_falling_path_sum.dart)
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- [**198.** House Robber](HouseRobber/house_robber.dart)
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- [**1143.** Longest Common Subsequence](LongestCommonSubsequence/longest_common_subsequence.dart)
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- [**150.** Evaluate Reverse Polish Notation](EvaluateReversePolishNotation/evaluate_reverse_polish_notation.dart)
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## Reach me via
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