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.

这道题需要注意的一点是当加到最后一位，carry还等于1时，需要在result 字符串的头加个‘1’。可以多运用if 的简写形式使代码简洁。代码如下：

public class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();
        int carry = 0;
        for (int i = num1.length() - 1, j = num2.length() - 1; i >= 0 || j >= 0 || carry == 1; i --, j --) {
            int n1 = i >= 0? num1.charAt(i) - '0': 0;
            int n2 = j >= 0? num2.charAt(j) - '0': 0;
            sb.append((n1 + n2 +carry) % 10);
            carry = (n1 + n2 +carry) / 10;
        }
        return sb.reverse().toString();
    }
}