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 {    int[] nums;    Random r;        public Solution(int[] nums) {        this.nums = nums;        r = new Random();    }        public int pick(int target) {        int res = -1;        int count = 0;        for (int i = 0; i < nums.length; i++) {            if (nums[i] != target) continue;            count++;            if (r.nextInt(count) == 0) {                res = i;            }        }        return res;    }}

本题用的是水塘抽樣，这是一种随即算法，且nextInt表示第一个元素一定会被取到，而且被取到的概率是1/n，故而选==0是一定会被抽中的