[LeetCode] 415. Add Strings

Problem

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.

Solution

class Solution {
    public String addStrings(String num1, String num2) {
        if (num1 == null || num1.length() == 0) return num2;
        if (num2 == null || num2.length() == 0) return num1;
        
        int i = num1.length()-1, j = num2.length()-1;
        StringBuilder sb = new StringBuilder();
        int carry = 0;
        while (i >= 0 && j >= 0) {
            int n1 = num1.charAt(i--)-'0';
            int n2 = num2.charAt(j--)-'0';
            System.out.println(n1+" "+n2);
            int sum = n1+n2+carry;
            sb.append(sum%10);
            carry = sum/10;
        }
        while (i >= 0) {
            int n = num1.charAt(i--)-'0';
            int sum = n+carry;
            sb.append(sum%10);
            carry = sum/10;
        }
        while (j >= 0) {
            int n = num2.charAt(j--)-'0';
            int sum = n+carry;
            sb.append(""+sum%10);
            carry = sum/10;
        }
        if (carry != 0) sb.append(carry);
        sb.reverse();
        return sb.toString();
    }
}