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.

求两个字符串的加法

    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();
        int i=num1.length()-1;
        int j=num2.length()-1;
        int carry = 0;
        while (i>=0 || j>=0 || carry==1)
        {
            int a= i>=0? num1.charAt(i--)-'0':0;
            int b = j>=0?num2.charAt(j--)-'0':0;
            int sum = a+b+carry;
            sb.insert(0,sum%10);
            carry = sum/10;
        }
        return sb.toString();
    }