5.cpp

/*

Random Pick with Weight
-----------------------

Given an array w of positive integers, where w[i] describes the weight of index i, write a function pickIndex which randomly picks an index in proportion to its weight.

Note:

1 <= w.length <= 10000
1 <= w[i] <= 10^5
pickIndex will be called at most 10000 times.
Example 1:

Input: 
["Solution","pickIndex"]
[[[1]],[]]
Output: [null,0]
Example 2:

Input: 
["Solution","pickIndex","pickIndex","pickIndex","pickIndex","pickIndex"]
[[[1,3]],[],[],[],[],[]]
Output: [null,0,1,1,1,0]
Explanation of Input Syntax:

The input is two lists: the subroutines called and their arguments. Solution's constructor has one argument, the array w. pickIndex has no arguments. Arguments are always wrapped with a list, even if there aren't any.
*/

class Solution {
public:
    vector <int> v;
    Solution(vector<int>& w) {
        v = w;
        srand(time(NULL));
          for(int i = 1; i < w.size(); i++){
             v[i] += v[i - 1];
          }
    }
    
    int pickIndex() {
        return upper_bound(v.begin(), v.end(), rand() % v.back()) - v.begin();
    }
};

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