#include <bits/stdc++.h>
using namespace std;
int sum,b;
int main(){
	int n;
    cin>>n;
	for(int i=1;i<=n;i++){
        b=i;
        while(b!=0){
        	if(b%10==1) sum+=1;
            b/=10;
        }
    }
    cout<<sum;
	
	return 0;
}


```cpp
#include<bits/stdc++.h>
using namespace std;
int main(){
    int n,ans = 0;
    cin>>n;
    for(int i = 1;i <= n;i++){
        int k = i;
        while(k > 0){
            if(k % 10 == 1) ans++;
            k /= 10;
        }
    }
    cout<<ans;
    return 0;
}
```


题解

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