Random Pick Index

Desicription

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:

1
2
3
4
5
6
7
8
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);

Solution

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Solution {
private:
std::map<int, std::vector<int>> map = std::map<int, std::vector<int>>{};
public:
Solution(const std::vector<int>& nums) {
for(int i = 0;i < nums.size(); i++) {
map[nums[i]].push_back(i);
}
}

int pick(int target) {
auto it = map.find(target);
if(it == map.end()) {
return 0;
}
return (*it).second[rand() % (*it).second.size()];
}
};

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