380.py

'''
	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();
'''

class RandomizedSet(object):

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

    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.positions:
        	self.values.append(val)
        	self.positions[val] = len(self.values) - 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.positions:
        	valIdx, lastEle = self.positions[val], self.values[-1]
        	self.positions[lastEle], self.values[valIdx] = valIdx, lastEle
        	self.values.pop()
        	self.positions.pop(val, 0)
        	return True
        return False

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


# 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()