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);

这类题目都类似，最关键的是理解

            if (rnd.nextInt(++count) == 0)                result = i;

这一段为什么等于从所有的target里随意的取一个

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