ValidateStackSequences946.kt

package medium

import java.util.*

/**
 * Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on an initially empty stack, or false otherwise.



Example 1:

Input: pushed = [1,2,3,4,5], popped = [4,5,3,2,1]
Output: true
Explanation: We might do the following sequence:
push(1), push(2), push(3), push(4),
pop() -> 4,
push(5),
pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1
Example 2:

Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2]
Output: false
Explanation: 1 cannot be popped before 2.


Constraints:

1 <= pushed.length <= 1000
0 <= pushed[i] <= 1000
All the elements of pushed are unique.
popped.length == pushed.length
popped is a permutation of pushed.
 */
fun validateStackSequences(pushed: IntArray, popped: IntArray): Boolean {
    val stack = Stack<Int>()
    var i = 0

    pushed.forEach { num ->
        stack.push(num)
        while (stack.isNotEmpty() && i < popped.size && popped[i] == stack.peek()) {
            stack.pop()
            i++
        }
    }
    return stack.isEmpty()
}