题目描述：

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:
The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

代码实现：

``````class Solution {
public:
string addStrings(string num1, string num2) {
string str;
int index1=num1.size()-1;
int index2=num2.size()-1;
int flag=0;
int num;
while(index1>=0&&index2>=0){
num=(num1[index1--]-'0')+(num2[index2--]-'0')+flag;
if(num>9){
flag=1;
str.append(to_string(num%10));
}else{
flag=0;
str.append(to_string(num));
}
}
while(index1>=0){
num=num1[index1--]-'0'+flag;
if(num>9){
flag=1;
str.append(to_string(num%10));
}else{
flag=0;
str.append(to_string(num));
}
}
while(index2>=0){
num=num2[index2--]-'0'+flag;
if(num>9){
flag=1;
str.append(to_string(num%10));
}else{
flag=0;
str.append(to_string(num));
}
}
if(flag==1){
str.append(to_string(1));
}
reverse(str.begin(),str.end());
return str;
}
};
``````