Given two non-negative numbers 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.

这道题让我们求两个字符串的相加，之前LeetCode出过几道类似的题目，比如二进制数相加，还有链表相加，或是字符串加1，基本思路很类似，都是一位一位相加，然后算和算进位，最后根据进位情况看需不需要补一个高位，难度不大，参见代码如下：

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

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Add Digits

Add Binary

Add Two Numbers

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[LeetCode] Add Strings 字符串相加