How do you solve Calculate Digit Sum of a String on LeetCode?

Calculate Digit Sum of a String (LeetCode #2243) 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: The string is processed in rounds, reducing its length each time.

What is the brute force solution for Calculate Digit Sum of a String?

The brute force approach for Calculate Digit Sum of a String is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach simulates the entire process of grouping and summing the digits repeatedly until the string's length is less than or equal to k. It is straightforward but inefficient for larger inputs. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Calculate Digit Sum of a String?

Calculate Digit Sum of a String uses the following concepts: String, Simulation. The recommended patterns to study are: String Manipulation, Simulation.

What are the interview tips for Calculate Digit Sum of a String?

Explain your thought process clearly as you write code. Consider edge cases, such as when the input string is already short enough. Optimize your solution after the brute force implementation.

What are common mistakes when solving Calculate Digit Sum of a String?

Not handling the last group correctly if it has fewer than k digits. Overcomplicating the string manipulation, leading to inefficiencies.

#2243

Calculate Digit Sum of a String

Easy

StringSimulationString ManipulationSimulation

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

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

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Intuition

Time O(n)Space O(n)

The optimal solution follows the same logic as the brute force but focuses on minimizing unnecessary operations and string manipulations. By directly calculating the digit sums and building the new string efficiently, we achieve better performance.

⚙️

Algorithm

3 steps

1Step 1: Use a loop to process the string until its length is less than or equal to k.
2Step 2: For each iteration, create a new string to hold the digit sums.
3Step 3: Use a single pass to calculate the sums of the groups and build the new string.

solution.py8 lines

1def digit_sum(s, k):
2    while len(s) > k:
3        new_s = ''
4        for i in range(0, len(s), k):
5            group_sum = sum(int(d) for d in s[i:i+k])
6            new_s += str(group_sum)
7        s = new_s
8    return s

ℹ

Complexity note: The time complexity is O(n) because we process each character in the string a limited number of times, while the space complexity is O(n) due to the new string being constructed.

1The string is processed in rounds, reducing its length each time.
2The digit sums are calculated for groups of size k, which can vary in the last round.

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