398.Random Pick Index

Tags: [random], [trick]

Link: https://leetcode.com/problems/random-pick-index/?tab=Description

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.

Note:
The array size can be very large. Solution that uses too much extra space will not pass the judge.

Example:

int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);

// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);

// pick(1) should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(1);

Solution:

import random

class Solution(object):

    def __init__(self, nums):
        """

        :type nums: List[int]
        :type numsSize: int
        """
        self.nums = nums

    def pick(self, target):
        """
        :type target: int
        :rtype: int
        """
        indexes = []
        for i in xrange(len(self.nums)):
            if self.nums[i] == target:
                indexes.append(i)

        rand_index = random.randint(0, len(indexes) - 1)
        return indexes[rand_index]


# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.pick(target)

Note:

• Time complexity = O(n), n is the number of elements of the given array.
• In Python, random.randint(x, y) will generate a random int in the range [x, y], both x and y are inclusive.
• In Python, random.randrange(x, y) will generate a random int in the range [x, y), x is inclusive, but y is exclusive.

Memory Limited Exceeded

import random

class Solution(object):

    def __init__(self, nums):
        """

        :type nums: List[int]
        :type numsSize: int
        """
        self.num_index_map = {}
        for i in xrange(len(nums)):
            num = nums[i]
            if num in self.num_index_map:
                self.num_index_map[num].append(i)
            else:
                self.num_index_map[num] = [i];

    def pick(self, target):
        """
        :type target: int
        :rtype: int
        """
        indexes = self.num_index_map[target]
        rand_index = random.randint(0, len(indexes) - 1)
        return indexes[rand_index]


# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.pick(target)

Note:

• If we do many times 'pick' operations, this solution is faster, but it uses more memory than the first accepted solution.