【两次过】Lintcode 655. Add Strings

以字符串的形式给出两个非负整数 num1 和 num2，返回 num1 和 num2 的和。

样例

给定 num1 = "123"，num2 = "45"
返回 "168"

解题思路：

    按位相加即可。注意最后的进位需要考虑。

class Solution {
public:
    /**
     * @param num1: a non-negative integers
     * @param num2: a non-negative integers
     * @return: return sum of num1 and num2
     */
    string addStrings(string &num1, string &num2) 
    {
        // write your code here
        if(num1.empty() || num2.empty())
            return "";
        
        int n1 = num1.size()-1;
        int n2 = num2.size()-1;
        int cnt = 0;
        
        string res;
        while(n1>=0 || n2>=0)
        {
            int a = (n1>=0)?num1[n1]-'0':0;
            int b = (n2>=0)?num2[n2]-'0':0;
                
            int tmp = a + b + cnt;
            
            res += tmp%10 + '0';
            cnt = tmp/10;
            
            n1--;
            n2--;
        }

        if(cnt)
            res += '1';
        
        reverse(res.begin(),res.end());
        
        return res;
    }
};