Random Pick Index

Problem Statement

Given an integer array nums 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.

Implement the Solution class:

• Solution(int[] nums) Initializes the object with the array nums.
• int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i's, then each index should have an equal probability of returning.

Leetcode Link

Example 1:

Input
["Solution", "pick", "pick", "pick"]
[[[1, 2, 3, 3, 3]], [3], [1], [3]]
Output
[null, 4, 0, 2]

Explanation
Solution solution = new Solution([1, 2, 3, 3, 3]);
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(1); // It should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.

Constraints:

• 1 <= nums.length <= 2 * 10⁴
• -2³¹ <= nums[i] <= 2³¹ - 1
• target is an integer from nums.
• At most 10⁴ calls will be made to pick.

Code

Python Code

import random
class Solution:

    def __init__(self, nums):
        """
        :type nums: List[int]
        """
        self.nums = nums

    def pick(self, target):
        """
        :type target: int
        :rtype: int
        """
        index, count = -1, 0
        for i in range(len(self.nums)):
            if self.nums[i] == target:
                if random.randint(0, count) == 0: index = i
                count += 1

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