【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.

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);

public class Solution {        private int[] n;    private Random r;        public Solution(int[] nums) {        n = nums;        r = new Random();    }        public int pick(int target) {        int result = -1;        int count = 0;        for (int i = 0; i < n.length; i++) {            if (n[i] != target)                continue;            if (r.nextInt(++count) == 0)                result = i;        }        return result;    }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(nums); * int param_1 = obj.pick(target); */