Day-05-Random Pick with Weight.cpp

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;
    int sum=0;
    Solution(vector<int>& w) {
      for(auto x: w){
          v.push_back(sum+x);
          sum+=x;
      }
    }

    int pickIndex() {
       int randWeight=rand()%sum;
        return upper_bound(v.begin(), v.end(), randWeight) - v.begin();
    }
};

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