4012. Count of Unfinished Tasks After Each Shift

You are given two integer arrays tasks and shifts.

• tasks[i] represents the time required to complete the i_th task.
• shifts[j] represents the amount of time available during the j^th shift.

The tasks must be processed in order from left to right.

• Carry-over: If a task is not completed during a shift, processing continues from the same point in that task during the next shift.
• Restart: If all tasks are completed during a shift, the shift ends immediately. Any unused time in that shift is discarded, and the next shift begins again from task 0.

A task is unfinished if it has not been fully completed. This includes a task that is currently in progress.

Return an integer array ans where ans[j] is the number of unfinished tasks immediately after the j^th shift.

class Solution {
public:
    vector<int> countTasks(vector<int>& tasks, vector<int>& shifts) {
        map<long long,int> ord{{0,tasks.size()}};
        for(long long i = 0, c = 0; i < tasks.size(); i++) {
            c += tasks[i];
            ord[c] = tasks.size() - i - 1;
        }
        vector<int> res;
        long long t = 0;
        for(auto& s : shifts) {
            t += s;
            res.push_back(prev(ord.upper_bound(t))->second);
            if(res.back() == 0) t = 0;
        }
        return res;
    }
};