Added question 380.

Author: luceCoding

Diff for: leetcode/medium/380_insert_delete_getRandom_O(1).md

@@ -0,0 +1,59 @@
+# 380. Insert Delete GetRandom O(1)
+
+## Solution
+- Run-time: O(1)
+- Space: O(N)
+- N = Number of values
+
+When we insert or delete at O(1), we think of a dictionary.
+When we getRandom() at O(1) we think of a randomized index from an array.
+If we merge the two, we can achieve O(1).
+We just have to do some clever swapping and popping from the last element of the array.
+
+If we use the dictionary to store the value's index in relation to the array.
+When we insert, we can insert the new value to the end of the array and keep its value to index relationship in the dictionary.
+
+When it comes to removing the value, we can fetch the value's corresponding index then swap it with the last element in the array.
+Then we can just pop that element from the array.
+This will help us achieve O(1) run-time.
+
+```
+class RandomizedSet:
+
+    def __init__(self):
+        """
+        Initialize your data structure here.
+        """
+        self.val_to_idx = dict()
+        self.values = list()
+
+    def insert(self, val: int) -> bool:
+        """
+        Inserts a value to the set. Returns true if the set did not already contain the specified element.
+        """
+        if val in self.val_to_idx:
+            return False
+        self.values.append(val)
+        self.val_to_idx[val] = len(self.values) - 1
+        return True
+
+
+    def remove(self, val: int) -> bool:
+        """
+        Removes a value from the set. Returns true if the set contained the specified element.
+        """
+        if val not in self.val_to_idx:
+            return False
+        idx = self.val_to_idx[val]
+        self.val_to_idx[self.values[-1]] = idx
+        self.values[idx], self.values[-1] = self.values[-1], self.values[idx]
+        del self.val_to_idx[val]
+        self.values.pop()
+        return True
+
+    def getRandom(self) -> int:
+        """
+        Get a random element from the set.
+        """
+        return self.values[random.randrange(0, len(self.values))]
+```