[LeetCode] 380. Insert Delete GetRandom O (1)

Problem : https://leetcode.com/problems/insert-delete-getrandom-o1/ 

The crux is using the last element of the list to replace the element needs to be removed to meet the requirement of O(1) deleting operation.

import random

class RandomizedSet:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        
        self.dict = {}
        self.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.
        """
        
        res = val not in self.dict
        
        if res:
            self.dict[val] = len(self.list)
            self.list.append(val)
        return res

    def remove(self, val: int) -> bool:
        """
        Removes a value from the set. Returns true if the set contained the specified element.
        """
        res = val in self.dict
        if res:
            # use the last element to replace the removed element
            index = self.dict[val]
            del self.dict[val]
            
            if index != len(self.list) - 1:
                self.list[index] = self.list[-1]
        
                # update element index and popup the last element from list
                self.dict[self.list[index]] = index
                
            self.list.pop()
        return res
        

    def getRandom(self) -> int:
        """
        Get a random element from the set.
        """
        
        pick = random.randint(0, len(self.list) - 1)
        
        return self.list[pick]
        


# Your RandomizedSet object will be instantiated and called as such:
# obj = RandomizedSet()
# param_1 = obj.insert(val)
# param_2 = obj.remove(val)
# param_3 = obj.getRandom()