Insert Delete GetRandom O(1) - Duplicates allowed.py

'''
Design a data structure that supports all following operations in average O(1) time.
Note: Duplicate elements are allowed.

    insert(val): Inserts an item val to the collection.
    remove(val): Removes an item val from the collection if present.
    getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

// Init an empty collection.
RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();

'''

class RandomizedCollection(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.val = []
        self.idx = {}
        
    def insert(self, val):
        """
        Inserts a value to the collection. Returns true if the collection did not already contain the specified element.
        :type val: int
        :rtype: bool
        """
        if val in self.idx:
            self.val.append(val)
            self.idx[val].add(len(self.val)-1)
            return False
        else:
            self.val.append(val)
            self.idx[val] = set([len(self.val)-1])
            return True
        

    def remove(self, val):
        """
        Removes a value from the collection. Returns true if the collection contained the specified element.
        :type val: int
        :rtype: bool
        """
        if val in self.idx:
            if val == self.val[-1]:
                val_idx = len(self.val)-1
                
                self.val.pop()
                self.idx[val].remove(val_idx)
                if not self.idx[val]:
                    del self.idx[val]
            else:
                last_val = self.val[-1]
                last_idx = len(self.val)-1
                val_idx = max(self.idx[val])
                
                self.val[val_idx], self.val[last_idx] = self.val[last_idx], self.val[val_idx]
                self.val.pop()
                
                self.idx[last_val].remove(last_idx)
                self.idx[val].remove(val_idx)
                self.idx[last_val].add(val_idx)
                if not self.idx[val]:
                    del self.idx[val]
            

            return True
        else:
            return False

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


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