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Insert Delete Getrandom O1

Created: March 21, 2020 by [lek-tin]

Last updated: March 21, 2020

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

from random import choice

class RandomizedSet:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.indexMap = {}
        self.vals = []

    def insert(self, val: int) -> bool:
        """
        Inserts a value to the collection. Returns true if the collection did not already contain the specified element.
        """
        if val in self.indexMap:
            return False

        index = len(self.vals)
        self.indexMap[val] = index
        self.vals.append(val)

        return True

    def remove(self, val: int) -> bool:
        """
        Removes a value from the collection. Returns true if the collection contained the specified element.
        """
        if val not in self.indexMap:
            return False

        lastVal = self.vals[-1]
        # get the index to remove from indexMap
        removeIndex = self.indexMap[val]
        # replace removeIndex index with lastVal
        self.vals[removeIndex] = lastVal
        self.indexMap[lastVal] = removeIndex
        self.vals.pop()
        # delete val from indexMap
        # need to delete at last because of edge case like: {0: 0} [0] where removeIndex = lastIndex
        del self.indexMap[val]
        return True

    def getRandom(self) -> int:
        """
        Get a random element from the collection.
        """
        return choice(self.vals)


# 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()