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 rand;    public Solution(int[] nums) {        this.nums=nums;        this.rand = new Random();    }    public int pick(int target) {        int total = 0;        int res = -1;        for(int i=0;i<nums.length;i++){            if(nums[i]==target){                int x = rand.nextInt(++total);                res = x==0?i:res;            }        }        return res;    }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(nums); * int param_1 = obj.pick(target); */