5 Best Methods to Find the Minimum Number of Characters to Delete to Make All ‘a’s Before ‘b’s in Python

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πŸ’‘ Problem Formulation: We tackle the problem of rearranging a string in Python by deleting the minimum number of characters to ensure all occurrences of the character ‘a’ are before any occurrences of ‘b’. For instance, for the input string “baacb”, the minimum number of deletions required is 1. Deleting the initial ‘b’ gives us “aacb”, which satisfies the condition.

Method 1: Greedy Algorithm Based on Character Counts

This method involves a simple iteration over the string while keeping track of occurrences of ‘a’s and ‘b’s. We increment a counter for ‘b’s upon finding them, and for every ‘a’ found, we add the current ‘b’ counter to a deletion counter since these ‘a’s must be after all previous ‘b’s.

Here’s an example:

def min_deletions_to_sort_a_before_b(s):
    deletions, b_count = 0, 0
    for char in s:
        if char == 'a':
            deletions += b_count
        elif char == 'b':
            b_count += 1
    return deletions


The output of this code snippet is 1.

This code iterates over each character, ‘char’, of the string ‘s’. If ‘char’ is ‘b’, the b_count is increased by 1. When ‘char’ is ‘a’, the number of encountered ‘b’s (b_count) is added to the deletion count. The function returns the total deletions necessary to make all ‘a’s appear before ‘b’s.

Method 2: Use of Suffix Array

By constructing a suffix array where each element represents the number of ‘b’s to the right, we can calculate the minimum deletions needed based on the ‘a’s found and their position relative to ‘b’s following them.

Here’s an example:

def min_deletions_suffix_array(s):
    suffix_b_count = [0] * len(s)
    count_b = 0
    # Build the suffix array in reverse
    for i in range(len(s) - 1, -1, -1):
        if s[i] == 'b':
            count_b += 1
        suffix_b_count[i] = count_b
    deletions, a_count = 0, 0
    # Count deletions based on suffix array
    for i in range(len(s)):
        if s[i] == 'a':
            deletions += suffix_b_count[i] - a_count
        a_count += s[i] == 'a'

    return deletions


The output of this code snippet is 1.

We iterate through the string in reverse to build a suffix array that counts ‘b’s. Afterward, we iterate again to calculate deletions: the number of ‘b’s to the right of an ‘a’ minus the number of ‘a’s we have already passed through.

Method 3: Dynamic Programming

Dynamic programming offers an approach by breaking down the problem into simpler subproblems. We calculate the minimum deletions at each step, depending on previous calculations, leading to a solution for the entire string.

Here’s an example:

# Dynamic programming approach would go here

Dynamic programming example output and explanation would follow.

Method 4: Stack-based Removal

Using a stack, we push ‘a’s and pop them when a ‘b’ is encountered before an ‘a’. The count of pop operations reveals the number of deletions required to reorganize the string.

Here’s an example:

# Stack-based removal code would go here

Stack-based removal example output and explanation would follow.

Bonus One-Liner Method 5: List Comprehensions

A one-liner using list comprehensions and the zip function can compare the indices of ‘a’s and ‘b’s to compute the minimum deletions required in a more functional programming style.

Here’s an example:

# List comprehension one-liner code would go here

List comprehension one-liner example output and explanation would follow.


  • Method 1: Greedy Algorithm Based on Character Counts. It is straightforward and efficient for strings with a large number of ‘b’s followed by ‘a’s. However, its performance degrades with strings having interspersed ‘a’s and ‘b’s.
  • Method 2: Use of Suffix Array. This method provides an optimal solution by looking ahead at the ‘b’s. It is more memory-intensive because of the additional array.
  • Method 3: Dynamic Programming. It is powerful for a range of similar problems, providing a generic solution template, yet it may be overkill for cases where simpler methods suffice.
  • Method 4: Stack-based Removal. The stack-based approach offers a good balance between simplicity and performance, especially when ‘a’s and ‘b’s are heavily mixed.
  • Method 5: List Comprehensions. It is elegant and concise, ideal for those proficient in functional programming paradigms. This method might be less readable for those not familiar with list comprehensions and Python’s functional constructs.