5.Randow 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 {
    vector<int>ps;
    int ts;
public:
    Solution(vector<int>& w) {
        ts=0;
        for(int i=0;i<w.size();i++){
            ts+=w[i];
            ps.push_back(ts);
        }
    }

    int pickIndex() {
        int i=rand()%ts;
        vector<int>::iterator upper;
    upper = lower_bound (ps.begin(), ps.end(), i);
        return upper-ps.begin();
    }
};