LeetCode 415 Add Strings

Problem：

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

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.

Summary:

以字符串的形式求两个整形数相加。

Analysis:

这道题是一道常规题，并不难，只需一位一位相加，注意在有进位的时候加上进位即可。

class Solution {
public:
    string addStrings(string num1, string num2) {
        int len1 = num1.size(), len2 = num2.size();
        int i = len1 - 1, j = len2 - 1, k = 0, carry = 0;
        char res[5200];
        
        while (i >= 0 && j >= 0) {
            int sum = num1[i] + num2[j] - 2 * '0' + carry;
            if (sum > 9) {
                sum -= 10;
                carry = 1;
            }
            else {
                carry = 0;
            }
            
            res[k++] = sum + '0';
            i--; j--;
        }
        
        while (i >= 0) {
            int sum = num1[i] + carry - '0';
            if (sum > 9) {
                sum -= 10;
                carry = 1;
            }
            else {
                carry = 0;
            }
            
            res[k++] = sum + '0';
            i--;
        }
        
        while (j >= 0) {
            int sum = num2[j] + carry - '0';
            if (sum > 9) {
                sum -= 10;
                carry = 1;
            }
            else {
                carry = 0;
            }
            
            res[k++] = sum + '0';
            j--;
        }
        
        if (carry) {
            res[k++] = '1';
        }
        
        res[k++] = '';
        
        for (i = 0; i < k / 2; i++) {
            char tmp = res[i];
            res[i] = res[k - i - 2];
            res[k - i - 2] = tmp;
        }
        
        string resStr = string(res);
        
        return resStr;
    }
};

以上为我写的原始代码，代码过于冗长，下面是参考网上大牛的代码后优化的代码，代码简介了很多：

class Solution {
public:
    string addStrings(string num1, string num2) {
        int len1 = num1.size(), len2 = num2.size();
        int i = len1 - 1, j = len2 - 1, carry = 0;
        string res = "";
        
        while (i >= 0 || j >= 0) {
            int sum1 = i >= 0 ? num1[i--] - '0' : 0;
            int sum2 = j >= 0 ? num2[j--] - '0' : 0;
            
            int sum = sum1 + sum2 + carry;
            res.insert(res.begin(), sum % 10 + '0');
            carry = sum / 10;
        }
        
        return carry ? "1" + res : res;
    }
};