0528. Random Pick with Weight.md

528. Random Pick with Weight, medium

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.

// 140ms, 81%
class Solution {
    vector<int> psum;
public:
    Solution(vector<int>& w) {
        psum.reserve(w.size());
        partial_sum(w.begin(), w.end(), back_inserter(psum));
        srand(time(NULL));
    }
    
    int pickIndex() {
        int v = rand() % psum.back();
        int lo = 0, hi = psum.size()-1;
        while (lo < hi) {
            int mid = lo + (hi - lo)/2;
            if (v >= psum[mid])
                lo = mid + 1;
            else
                hi = mid;
        }
        return lo;
    }
};

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