## 398. RandomPick 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 is equal to 1.
solution.pick(1);
``````
``````class Solution {
vector<int> nums;
public:
Solution(vector<int> nums) {
this->nums = nums;
}

int pick(int target) {
int res;
int cnt = 0;
for(int i = 0; i < nums.size(); i++){
if(nums[i] != target) continue;
if(cnt == 0) {res = i; cnt++;}
else{
// with prob 1/(n+1) to replace the previous index
if(rand()%(++cnt) == 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);
*/
``````
``````class Solution {
int[] nums;
Random rand;
public Solution(int[] nums) {
this.nums = nums;
rand = new Random();
}

public int pick(int target) {
int cnt = 0, res = -1;
for(int i = 0; i < nums.length; i++){
if(nums[i] != target) continue;
// with prob 1/(n+1) to replace the previous index
if(rand.nextInt(++cnt) == 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);
*/
``````