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.

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


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.

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

Reservoir Sampling

public class Solution {
    int[] nums;
    Random r;

    public Solution(int[] nums) {
        this.nums = nums;
        r = new Random();
    public int pick(int target) {
        int idx = 0;
        int count = 0;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != target) continue;
// for the nth target, the prob of returning is 1/n
// for the (n-1)th target, the prob of returning is (n-1)/n * 1/(n-1) = 1/n
            if (r.nextInt(++count) == 0) {
                idx = i; 
        return idx;