为什么不行啊

代码有点*山（）但是感觉能实现功能，为什么不行呢

#include<bits/stdc++.h>
using namespace std;
string add(string n,string m){
    int len = max(n.length(),m.length());
    
    if (n[0]-'0'+m[0]-'0'>=10)len++;
    while (m.length()<len)m='0'+m;
    while (n.length()<len)n='0'+n;
    int carry =0;
    string s(len, '0');
    for (int i=len-1 ; i>=0 ; i--){
        

        int a = n[i]-'0',b = m[i]-'0';
        

        int j = a+b+carry;
        carry=j/10;
        s[i]=j%10+'0';
        //cout << carry << '-' << a << ' '<< b <<' '<<  j << endl;

        
    }
    int k = 0;
    if (carry > 0) {
        s = to_string(carry) + s;
    }
    if (s[0]=='0') k = 1;
    for (int i = k;i<=len-1; i++){
    cout << s[i];
    
    }
    return s;
} 
int main(){
    string a,b;
    cin >> a >> b;
    add(a,b);
    return 0;
}