How do you solve Add Strings on LeetCode?

Add Strings (LeetCode #415) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Simulating addition helps manage carry effectively.

What is the brute force solution for Add Strings?

The brute force approach for Add Strings is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach involves converting each character of the string representations of the numbers into their integer values, summing them up, and then converting the result back to a string. This method is straightforward but inefficient for large numbers. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Add Strings?

Add Strings uses the following concepts: Math, String, Simulation. The recommended patterns to study are: Array, String Manipulation.

What are the interview tips for Add Strings?

Explain your thought process clearly as you code. Consider edge cases, such as leading zeros or empty strings. Practice simulating the addition process on paper before coding.

What are common mistakes when solving Add Strings?

Forgetting to handle the carry after the last digit. Not considering the case where one string is longer than the other.

#415

Add Strings

Easy

MathStringSimulationArrayString Manipulation

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

The optimal solution simulates the addition process digit by digit, similar to how we add numbers on paper. We start from the rightmost digit and handle carry-over, ensuring we don't convert the entire strings to integers.

⚙️

Algorithm

4 steps

1Step 1: Initialize two pointers at the end of both strings and a carry variable set to 0.
2Step 2: Loop until both pointers are valid or there is a carry.
3Step 3: Convert the current digits to integers, add them along with the carry, and update the result string.
4Step 4: Move the pointers left and repeat until all digits are processed.

solution.py15 lines

1# Full working Python code
2
3def addStrings(num1, num2):
4    result = []
5    i, j, carry = len(num1) - 1, len(num2) - 1, 0
6    while i >= 0 or j >= 0 or carry:
7        x1 = int(num1[i]) if i >= 0 else 0
8        x2 = int(num2[j]) if j >= 0 else 0
9        carry, digit = divmod(x1 + x2 + carry, 10)
10        result.append(str(digit))
11        i -= 1
12        j -= 1
13    return ''.join(result[::-1])
14
15print(addStrings('11', '123'))

ℹ

Complexity note: This complexity is O(n) because we process each digit of the input strings exactly once. The space complexity is O(n) due to the storage of the result string.

1Simulating addition helps manage carry effectively.
2Working from the least significant digit to the most significant is crucial.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.