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.

两个字符串求和

做出来了 但是不够简洁

discuss里有一个比较优雅 

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 x = i < 0 ? 0 : num1.charAt(i) - '0';            int y = j < 0 ? 0 : num2.charAt(j) - '0';            sb.append((x + y + carry) % 10);            carry = (x + y + carry) / 10;        }        return sb.reverse().toString();    }}