415. Add Strings

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
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

 1     public String addStrings(String num1, String num2) {
 2         StringBuilder sb = new StringBuilder();
 3         int i=num1.length()-1;
 4         int j=num2.length()-1;
 5         int carry = 0;
 6         while (i>=0 || j>=0 || carry==1)
 7         {
 8             int a= i>=0? num1.charAt(i--)-'0':0;
 9             int b = j>=0?num2.charAt(j--)-'0':0;
10             int sum = a+b+carry;
11             sb.insert(0,sum%10);
12             carry = sum/10;
13         }
14         return sb.toString();
15     }