题解

#include <bits/stdc++.h>
using namespace std;
int main(){
    int day,coins;
    cin >> day;
    int lgs = 0;
    int step = 1;
    for(int i=1;i<=day;i++){
        coins += step;
        if(step+lgs==i){
            lgs = step+lgs;
            step+=1;
        }
    }
    cout << coins;
    return 0;
}

代码太粗劣了，写成O(N)