Leetcode-398. Random Pick Index

前言：为了后续的实习面试，开始疯狂刷题，非常欢迎志同道合的朋友一起交流。因为时间比较紧张，目前的规划是先过一遍，写出能想到的最优算法，第二遍再考虑最优或者较优的方法。如有错误欢迎指正。博主首发CSDN，mcf171专栏。这次比赛略无语，没想到前3题都可以用暴力解。

博客链接：mcf171的博客

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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);

这是一个非常经典的蓄水池算法。Your runtime beats 91.32% of java submissions.

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;    }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(nums); * int param_1 = obj.pick(target); */