题解

#include <bits/stdc++.h>
using namespace std;
int main(){
    int n ,j ,sum = 0;
    cin >> n;
    for (int i = 1 ; i <= n ; i++){
        j = i;
        while (j){  //除一下，取余一下，获得每一数位的值
            if (j % 10 == 1)
                sum += 1;
            j /= 10;
        }
	}
    cout << sum;
    return 0;
}