[LeetCode] Problem 415 - Add Strings

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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.

Code

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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) {
        int a = i >= 0 ? num1.charAt(i--) - '0' : 0;
        int b = j >= 0 ? num2.charAt(j--) - '0' : 0;
        int sum = a + b + carry;

        carry = sum / 10;
        sb.append(sum % 10);
    }
    
    if (carry != 0)
        sb.append(carry);

    return sb.reverse().toString();
}