How do you solve Sum of Digits of String After Convert on LeetCode?

Sum of Digits of String After Convert (LeetCode #1945) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(1) space complexity. Key insight: The initial transformation creates a number that can be large, but the subsequent digit sums reduce it significantly.

What is the brute force solution for Sum of Digits of String After Convert?

The brute force approach for Sum of Digits of String After Convert is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach involves converting each character in the string to its corresponding number, concatenating those numbers into a single integer, and then repeatedly summing the digits k times. This approach is straightforward but can be inefficient due to the repeated summation of digits. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Sum of Digits of String After Convert?

Sum of Digits of String After Convert uses the following concepts: String, Simulation. The recommended patterns to study are: Hash Map, Array.

What are the interview tips for Sum of Digits of String After Convert?

Always clarify the input constraints and edge cases with the interviewer. Explain your thought process clearly while coding; it shows your understanding. After implementing, discuss potential optimizations and their implications.

What are common mistakes when solving Sum of Digits of String After Convert?

Not converting characters correctly to their corresponding integer values. Forgetting to handle the case when k = 1, leading to unnecessary calculations.

#1945

Sum of Digits of String After Convert

Easy

StringSimulationHash MapArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

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

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Intuition

Time O(n)Space O(1)

After the first transformation, the resulting number will be relatively small, allowing us to optimize the repeated summation of digits. Instead of performing the summation k times, we can directly compute the result using properties of digit sums.

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Algorithm

2 steps

1Step 1: Convert each character in the string to its corresponding position in the alphabet and calculate the initial digit sum.
2Step 2: If k > 1, calculate the final result using the properties of digit sums, which can be simplified to a single digit.

solution.py9 lines

1# Full working Python code
2
3def sum_of_digits(s, k):
4    # Step 1: Initial digit sum
5    initial_sum = sum(ord(char) - ord('a') + 1 for char in s)
6    # Step 2: Reduce the sum k times
7    for _ in range(k - 1):
8        initial_sum = sum(int(digit) for digit in str(initial_sum))
9    return initial_sum

ℹ

Complexity note: The time complexity is O(n) because we only need to iterate through the string once to compute the initial sum. The space complexity is O(1) since we are using a constant amount of space.

1The initial transformation creates a number that can be large, but the subsequent digit sums reduce it significantly.
2Understanding properties of digit sums can help optimize repeated operations.

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