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.

Solution:

This problem is similar to Add Binary.

We start from the end of the strings and go backward.



Code:

public class Solution {
    public String addStrings(String num1, String num2) {
        int len1 = num1.length() - 1;
        int len2 = num2.length() - 1;
        int carry = 0;
        StringBuilder sb = new StringBuilder();
        while (carry == 1 || len1 >= 0 || len2 >= 0) {
            int v1 = len1 >= 0 ? num1.charAt(len1--) - '0' : 0;
            int v2 = len2 >= 0 ? num2.charAt(len2--) - '0' : 0;
            int sum = v1 + v2 + carry;
            sb.append(sum % 10);
            carry = sum / 10;
        }
        return sb.reverse().toString();
    }
}