[Binary search] leetcode 528 Random Pick with Weight

problem:https://leetcode.com/problems/random-pick-with-weight/

        First, a cumulative frequency of calculation, according to the total number of random frequency, then find the corresponding random number to obtain the current index by half.

class Solution {
public:
    vector<int> sum;
    int count;
    unordered_map<int, int> mapIndex;
    Solution(vector<int>& w) {
        for (int i = 0; i < w.size(); i++)
        {
            if (w[i] != 0)
            {
                int num = sum.size() > 0 ? sum.back() + w[i] : w[i];
                sum.push_back(num);
                mapIndex[num] = i;
            }
        }
        count = sum.size() ? sum.back() : 0;
    }

    int pickIndex() {
        if (count == 0) return -1;
        int index = rand() % count;
        auto it = upper_bound(sum.begin(), sum.end(), index);
        return mapIndex[*it];

    }
};