# Random Pick Index Problem

## Description

LeetCode Problem 398.

Given an integer array nums 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.

Implement the Solution class:

• Solution(int[] nums) Initializes the object with the array nums.
• int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i’s, then each index should have an equal probability of returning.

Example 1:

``````1
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Input
["Solution", "pick", "pick", "pick"]
[[[1, 2, 3, 3, 3]], , , ]
Output
[null, 4, 0, 2]
Explanation
Solution solution = new Solution([1, 2, 3, 3, 3]);
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(1); // It should return 0. Since in the array only nums is equal to 1.
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
``````

Constraints:

• 1 <= nums.length <= 2 * 10^4
• -2^31 <= nums[i] <= 2^31 - 1
• target is an integer from nums.
• At most 10^4 calls will be made to pick.

## Sample C++ Code

``````1
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class Solution {
unordered_map<int, vector<int>> map;
public:
Solution(vector<int>& nums) {
for(int i=0;i<nums.size();i++){
map[nums[i]].push_back(i);
}
}

int pick(int target) {
vector<int> random = map[target];
return random[rand()%random.size()];
}
};

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