1894. Find the Student that Will Replace the Chalk

There are n students in a class numbered from 0 to n - 1. The teacher will give each student a problem starting with the student number 0, then the student number 1, and so on until the teacher reaches the student number n - 1. After that, the teacher will restart the process, starting with the student number 0 again.

You are given a 0-indexed integer array chalk and an integer k. There are initially k pieces of chalk. When the student number i is given a problem to solve, they will use chalk[i] pieces of chalk to solve that problem. However, if the current number of chalk pieces is strictly less than chalk[i], then the student number i will be asked to replace the chalk.

Return the index of the student that will replace the chalk.

class Solution {
public:
    int chalkReplacer(vector<int>& chalk, int k) {
        int n = chalk.size();
        vector<long long> psum(n, chalk[0]);
        for(int i = 1; i < n; i++) {
            psum[i] = psum[i-1] + chalk[i];
        }
        k %= psum.back();
        return upper_bound(begin(psum), end(psum), k) - begin(psum);
    }
};