380. Insert Delete GetRandom O(1)

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

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. 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: HashMap

import random


class RandomizedSet(object):
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.vals = list()
        self.pos = dict()

    def insert(self, val):
        """
        Inserts a value to the set. 
        Returns true if the set did not already contain the specified element.
        :type val: int
        :rtype: bool
        """
        if val not in self.pos:
            self.vals.append(val)
            self.pos[val] = len(self.vals) - 1
            return True
        return False

    def remove(self, val):
        """
        Removes a value from the set. 
        Returns true if the set contained the specified element.
        :type val: int
        :rtype: bool
        """
        if val in self.pos:
            pos = self.pos[val]
            last = self.vals[-1]
            self.vals[pos] = last
            self.pos[last] = pos
            self.vals.pop()
            del self.pos[val]
            return True
        return False

    def getRandom(self):
        """
        Get a random element from the set.
        :rtype: int
        """
        return random.choice(self.vals)

Lessons:

• If use set directly, getRandom won't be O(1), since you have to convert the whole set to tuple first, random.choice(tuple(set)).
• Use list to save values, use dictionary to save each value's position.

Related:

• 381. Insert Delete GetRandom O(1) - Duplicates allowed