415. Add Strings

注意进位:

class Solution {
public:
    string addStrings(string num1, string num2) {
        int i=num1.size()-1,j=num2.size()-1,flag=0;
        string res;
        while(i>=0&&j>=0)
        {
            int sum=(num1[i]-'0') + (num2[j]-'0') +flag;
            flag=sum/10;  //进位         
            sum %= 10;
            res = char(sum+'0') + res;
            i--;
            j--;
        }
        while(i>=0)
        {
            int sum=(num1[i]-'0') + flag;
            flag=sum/10;           
            sum %= 10;
            res = char(sum+'0') + res;
            i--;
        }
        while(j>=0)
        {
            int sum=(num2[j]-'0') +flag;
            flag=sum/10;           
            sum %= 10;
            res = char(sum+'0') + res;
            j--;
        }
        if(flag>0)
            res = char(flag + '0') + res;
        
        return res;
    }
};