ds_const_time_lc380.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();
import random


class RandomizedSet(object):
    """
    The idea is use a list to hold all the data,
    and use a dictionary(hash map) to remember where each val is (index)
    And to delete a val, simply get that index and swap it with the last val in list to pop
    """

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

    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 in self.index:
            return False
        self.index[val] = len(self.output)
        self.output.append(val)
        return True

    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 not in self.index:
            return False
        index = self.index[val]
        last = self.output[-1]
        self.index[last] = index
        self.output[index] = last
        self.output.pop()
        del self.index[val]
        return True

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