# 398 Random Pick Index

### Problem

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);
``````

### Solutions

``````class Solution {
private int[] nums;
public Solution(int[] nums) {
this.nums = nums;
}

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

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