Formatted question description: https://leetcode.ca/all/415.html

415. Add Strings (Easy)

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.

Companies:
Facebook, Microsoft

Related Topics:
Math

Similar Questions:

• Add Two Numbers (Medium)
• Multiply Strings (Medium)

Solution 1.

// OJ: https://leetcode.com/problems/add-strings/

// Time: O(MN)
// Space: O(1)
class Solution {
public:
    string addStrings(string num1, string num2) {
        string sum;
        int carry = 0;
        auto i1 = num1.rbegin(), i2 = num2.rbegin();
        while (i1 != num1.rend() || i2 != num2.rend() || carry) {
            int n = carry;
            if (i1 != num1.rend()) n += *i1++ - '0';
            if (i2 != num2.rend()) n += *i2++ - '0';
            carry = n / 10;
            sum += (n % 10) + '0';
        }
        reverse(sum.begin(), sum.end());
        return sum;
    }
};

Java

class Solution {
    public String addStrings(String num1, String num2) {
        if (num1.equals("0") && num2.equals("0"))
            return "0";
        int length1 = num1.length(), length2 = num2.length();
        while (num1.length() < length2)
            num1 = "0" + num1;
        while (num2.length() < length1)
            num2 = "0" + num2;
        int totalLength = Math.max(length1, length2) + 1;
        int[] sumArray = new int[totalLength];
        for (int i = totalLength - 1; i > 0; i--) {
            int digit1 = num1.charAt(i - 1) - '0';
            int digit2 = num2.charAt(i - 1) - '0';
            sumArray[i] += digit1 + digit2;
            if (sumArray[i] >= 10) {
                sumArray[i - 1] += sumArray[i] / 10;
                sumArray[i] %= 10;
            }
        }
        StringBuffer sb = new StringBuffer();
        if (sumArray[0] != 0)
            sb.append(sumArray[0]);
        for (int i = 1; i < totalLength; i++)
            sb.append(sumArray[i]);
        String sum = sb.toString();
        return sum;
    }
}

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