大数加法

#include <iostream>#include <cstdio>#include <cstring>#include <sstream>#include <algorithm>#include <vector>#include <cmath>using namespace std;#define mem(a,n) memset(a,sizeof(a),n)typedef long long ll;const int mod=1e9+7;const int N=1e5+5;const int INF=0x3f3f3f3f;string a,b,c,d,ans;string add(string s1,string s2){    string s("");    int len1=s1.size()-1;    int len2=s2.size()-1;    int ca=0;///ca表示进位    while(len1!=-1&&len2!=-1)    {        int sum=ca+(s1[len1--]-'0')+(s2[len2--]-'0');        s+=((sum)%10+'0');        ca=sum/10;    }    while(len1!=-1)///若len1!=-1    {        int sum=ca+(s1[len1--]-'0');        s+=((sum)%10+'0');        ca=sum/10;    }    while(len2!=-1)///若len2!=-1    {        int sum=ca+(s2[len2--]-'0');        s+=((sum)%10+'0');        ca=sum/10;    }    if(ca) s+=(ca+'0');    reverse(s.begin(),s.end());    return s;}int main(){    int T;    cin>>T;    while(T--)    {        cin>>a>>b>>c>>d;        ans=add(a,b);        ans=add(ans,c);        ans=add(ans,d);        cout<<ans<<endl;    }    return 0;}