415. Add Strings

Problem:

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.

思路：

Solution (C++):

string addStrings(string num1, string num2) {
    int m = num1.size() - 1, n = num2.size() - 1, carry = 0;
    string res = "";
    
    while (m >= 0 || n >= 0 || carry) {
        long long sum = 0;
        if (m >= 0)  { sum += (num1[m--]-'0'); }
        if (n >= 0)  { sum += (num2[n--]-'0'); }
        sum += carry;
        carry = sum / 10;
        sum %= 10;
        res += to_string(sum);
    }
    reverse(res.begin(), res.end());
    return res;
}

性能：

Runtime: 8 ms  Memory Usage: 6.9 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB