Leetcode -- Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

class Solution {
public:
string add(string s,string t)
{
int n1=s.size(),n2=t.size();
if(n1<n2) return add(t,s);
string res;
char cur;
t=string(n1-n2,'0')+t;
int c=0;
for(int i=n1-1;i>=0;--i)
{
int tmp=c+s[i]-'0'+t[i]-'0';
c=tmp/10;
tmp%=10;
cur=tmp+'0';
res=cur+res;
}
if(c>0)
{
cur=c+'0';
res=cur+res;
}
return res;
}
string multi(string s,char t)
{
int n=s.size(),c=0,v=t-'0';
string res;
char ch;
for(int i=n-1;i>=0;--i)
{
int val = (s[i]-'0')*v+c;
c=val/10;
val%=10;
ch = val+'0';
res=ch+res;
}
if(c>0)
{
ch=c+'0';
res=ch+res;
}
return res;
}
string multiply(string num1, string num2) {
int n1=num1.size(),n2=num2.size();
string res=multi(num1,num2[n2-1]);
for(int i=n2-2;i>=0;--i)
{
string cur=multi(num1,num2[i]);
cur+=string(n2-1-i,'0');
res=add(cur,res);
}
while(res.size()>1&&res[0]=='0') res=res.substr(1);
return res;
}
};