LeetCode 398. Random Pick Index

Problem Statement

(Source) 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

Tags: Reservoir Sampling.

If we can assume random.random() is real random generator…

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        """        res = None        y = -1        for i, num in enumerate(self.nums):            if num == target:                x = random.random()                if x > y:                    y = x                    res = i        return res# Your Solution object will be instantiated and called as such:# obj = Solution(nums)# param_1 = obj.pick(target)