[leetcode]415. Add Strings

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

Note:

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

分析：

将两个用字符串表示的非负整数相加，返回和的字符串，不能直接使用库函数。从num1和num2尾部开始依次取出数字与进位相加，用carry表示进位，每步求和结果从开始处依次插入res中。加到最后时判断进位是否为1，若是，在res前补“1”即可。

class Solution {
public:
    string addStrings(string num1, string num2) {
        int len1 = num1.size();
        int len2 = num2.size();
        int carry = 0;
        int i = len1-1;
        int j = len2-1;
        string res = "";
        while(i>=0 || j>=0)
        {
            int a = 0;
            int b = 0;
            if(i >= 0)
                a = num1[i] - '0';
            else
                a = 0; 
            if(j >= 0)
                b = num2[j] - '0';
            else
                b = 0;            
            i--;
            j--;
            int sum = a + b + carry;
            res.insert(res.begin() , sum%10+'0');
            carry = sum/10;
        }
        if(carry == 1)
            return "1" + res;
        else
            return res;        
    }
};