Reservoir Sampling.md

Reservoir Sampling

Leetcode 398. Random Pick Index

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.

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 :

class Solution {
    Random random;
    int[] nums;
    public Solution(int[] nums) {
        this.nums = nums;
        this.random = new Random();
    }
    
    public int pick(int target) {
        int res = -1, count = 0;
        for(int i = 0; i < nums.length; i++) {
            if(nums[i] == target) {
                res = random.nextInt(++count) == 0 ? i : res;
            } 
        }
        return res;
    }
}