415. Add Strings

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

You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.

按照正常加法计算方法即可。
先将指针设置在两个String的末尾。
每次计算位置上加和的结果。
最后需要将字符串翻转过来。

class Solution {
    public String addStrings(String num1, String num2) {
        int i = num1.length() - 1;
        int j = num2.length() - 1;
        int carry = 0;
        
        StringBuffer ans = new StringBuffer();
        
        while( i >= 0 || j >= 0 || carry > 0){
            int x = i >= 0 ? num1.charAt(i) - '0' : 0;
            int y = j >= 0 ? num2.charAt(j) - '0' : 0;
            int result = x + y + carry;
            
            ans.append(result % 10);
            carry = result / 10;
            
            i--;
            j--;
        }
        ans.reverse();
        return ans.toString();
    }
}