How do you solve Minimum Additions to Make Valid String on LeetCode?

Minimum Additions to Make Valid String (LeetCode #2645) 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: Understanding how to compare characters in a cyclic pattern is crucial.

What is the time complexity of Minimum Additions to Make Valid String?

The optimal solution for Minimum Additions to Make Valid String runs in O(n) time and uses O(1) space. The complexity is O(n) because we only make a single pass through the input string, making it much more efficient than the brute-force approach.

What is the brute force solution for Minimum Additions to Make Valid String?

The brute force approach for Minimum Additions to Make Valid String is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute-force approach involves checking each character of the input string against the expected characters of the pattern 'abc'. If they don't match, we count how many characters we need to insert to make the string valid. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Minimum Additions to Make Valid String?

Minimum Additions to Make Valid String uses the following concepts: String, Dynamic Programming, Stack, Greedy. The recommended patterns to study are: Two Pointers, Greedy.

What are the interview tips for Minimum Additions to Make Valid String?

Always clarify the problem statement and constraints before jumping into coding. Think about edge cases, such as strings that are already valid or those that require maximum additions. Practice explaining your thought process while coding to simulate the interview environment.

What are common mistakes when solving Minimum Additions to Make Valid String?

Students often forget to handle the wrap-around of the pattern pointer correctly. Some may confuse the conditions for matching and incrementing the additions count.

#2645

Minimum Additions to Make Valid String

Medium

StringDynamic ProgrammingStackGreedyTwo PointersGreedy

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

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

💡

Intuition

Time O(n)Space O(1)

The optimal approach uses a single pass through the string with two pointers, one for the input string and one for the pattern 'abc'. This allows us to efficiently count how many characters need to be added without redundant checks.

⚙️

Algorithm

5 steps

1Step 1: Initialize two pointers, one for the input string and one for the pattern 'abc'.
2Step 2: Loop through the input string, comparing each character with the current character in the pattern.
3Step 3: If the characters match, move both pointers forward.
4Step 4: If they don't match, increment the additions count and only move the pattern pointer forward.
5Step 5: After processing the input string, add any remaining characters needed from the pattern to the additions count.

solution.py12 lines

1def minAdditions(word):
2    pattern = 'abc'
3    i, j, additions = 0, 0, 0
4    while i < len(word):
5        if word[i] == pattern[j]:
6            j += 1
7        else:
8            additions += 1
9        i += 1
10        j %= 3
11    additions += (3 - j) % 3
12    return additions

ℹ

Complexity note: The complexity is O(n) because we only make a single pass through the input string, making it much more efficient than the brute-force approach.

1Understanding how to compare characters in a cyclic pattern is crucial.
2Recognizing when to increment the additions count helps streamline the solution.

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