# 380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(1) time.

insert(val): Inserts an item val to the set if not already present.
remove(val): Removes an item val from the set if present.
getRandom(): Returns a random element from current set of elements. Each element must have the same probability of being returned.

Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

# Solution

Approach 1: use the fact that array has O(1) random access, and can be resized to get amortized O(1) insert and delete.

# Code (Python)

Approach 1:

import random
class RandomizedSet:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.locations = {} # mapping of {value: index of the value as in self.values}
        self.values = [] # a python list is like an ArrayList in Java because it's represented as an array with size expansions/reductions; append() and poplast() are both O(1) amortized

    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.locations:
            return False
        self.values.append(val)
        self.locations[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.values:
            return False
        if len(self.values) == 1:
            self.locations = {}
            self.values = []
            return True
        
        # delete the value in array self.values, and use the last item of the array to fill in the hole
        hole_index = self.locations[val]
        self.locations.pop(val)
        swap_index = len(self.values) - 1
        swap_value = self.values[swap_index]
        self.values[hole_index] = swap_value
        self.locations[swap_value] = hole_index
        self.values.pop()
        return True

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

# Code (C++)

Approach 1:

Approach 2: