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);

思路：这题考察的就是水塘采样，就是target有很多个，然后在这些target中，随意挑选一个。算法就是：random.nextInt( count) 如果==0，那么就选当前的，否则还是之前的值不变。

public class Solution {    Random random = new Random();    int[] array;    public Solution(int[] nums) {        this.array = nums;    }        public int pick(int target) {       int result = 0;       int upbound = 1;       for(int i=0; i<this.array.length; i++){           if(array[i] == target){               if(random.nextInt(upbound) == 0){                   result = i;               }               upbound++;           }       }       return result;    }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(nums); * int param_1 = obj.pick(target); */