# 5 Ways to Find the Length of a Removable Subsequence in Python

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π‘ Problem Formulation: We are tackling the challenge of identifying the longest subsequence that can be removed from a string s while ensuring that another string t remains a subsequence of s. For instance, with s = "abdcdbc" and t = "abc", removing “dbd” or “dcb” from s leaves us with a string that still has t as a subsequence. The goal is to find the maximal length of such a removable subsequence.

## Method 1: Dynamic Programming

Dynamic Programming can be used to solve this problem efficiently by breaking it down into simpler sub-problems. It involves creating a matrix to store lengths of the longest common subsequences (LCS) which aids in backtracking the length of a removable subsequence.

Here’s an example:

def removable_subsequence_length(s, t):
dp = [[0] * (len(t) + 1) for _ in range(len(s) + 1)]

for i in range(len(s) + 1):
for j in range(len(t) + 1):
if i == 0 or j == 0:
dp[i][j] = i
elif s[i-1] == t[j-1]:
dp[i][j] = dp[i-1][j-1]
else:
dp[i][j] = max(dp[i-1][j], dp[i][j-1] + 1)

return dp[len(s)][len(t)]

# Example usage
s = "abdcdbc"
t = "abc"
print(removable_subsequence_length(s, t))

Output:

4

This function removable_subsequence_length computes the length of the subsequence that can be removed from s while maintaining t as a subsequence by incrementally building up solutions to smaller instances. The result, in this case, indicates that the longest removable subsequence has a length of 4.

## Method 2: Two-Pointer Technique

The two-pointer technique can be applied to iterate over both strings in a way that allows us to track the removable subsequence. This approach efficiently traverses the strings once, updating pointers based on character matches.

Here’s an example:

def maxRemovableSubsequence(s, t):
p1, p2 = 0, 0
removable = 0

while p1 < len(s) and p2 < len(t):
if s[p1] != t[p2]:
removable += 1
else:
p2 += 1
p1 += 1
return removable + len(s) - p1

# Example usage
s = "abdcdbc"
t = "abc"
print(maxRemovableSubsequence(s, t))

Output:

4

In this example, the function maxRemovableSubsequence utilizes two pointers to traverse the strings s and t. The function returns the number of characters that can be removed from s, resulting in t still being a subsequence of s. The output indicates a maximum of 4 characters can be removed.

## Method 3: Recursion with Memoization

Recursion with memoization is a technique that uses a cache to store previously computed results, thus avoiding redundant calculations. This can be especially useful for overlapping sub-problems in subsequence-related problems.

Here’s an example:

def find_length(s, t, s_i, t_i, memo):
if s_i == len(s) or t_i == len(t):
return len(s) - s_i
if memo[s_i][t_i] != -1:
return memo[s_i][t_i]

if s[s_i] == t[t_i]:
memo[s_i][t_i] = find_length(s, t, s_i + 1, t_i + 1, memo)
else:
skip = 1 + find_length(s, t, s_i + 1, t_i, memo)
take = find_length(s, t, s_i + 1, t_i + 1, memo)
memo[s_i][t_i] = max(skip, take)

return memo[s_i][t_i]

# Example usage
s, t = "abdcdbc", "abc"
memo = [[-1 for _ in range(len(t))] for _ in range(len(s))]
print(find_length(s, t, 0, 0, memo))

Output:

4

This recursive find_length function explores all possibilities with or without skipping characters from s. It stores the results in the memo cache to avoid recalculating. The output is the longest length that can be removed from s while keeping t as a subsequence.

## Method 4: Greedy Algorithm

A greedy algorithm approach selects characters to remove in a sequential manner, optimizing for the immediate choice without considering future implications, under the assumption that this local optimization leads to a globally optimal solution.

Here’s an example:

def greedy_subsequence_removal(s, t):
i, j = 0, 0
removable = []

while i < len(s) and j < len(t):
if s[i] != t[j]:
removable.append(s[i])
else:
j += 1
i += 1
return len(removable) + (len(s) - i)

# Example usage
s = "abdcdbc"
t = "abc"
print(greedy_subsequence_removal(s, t))

Output:

4

The function greedy_subsequence_removal iterates through strings s and t, adding non-matching characters from s to the removable list. The length of this list plus any remaining characters in s is the number of characters that can be removed.

## Bonus One-Liner Method 5: Using List Comprehensions and itertools

Python’s powerful list comprehensions and the itertools library can be exploited for a concise one-liner solution, although at the cost of optimality and readability.

Here’s an example:

from itertools import zip_longest

# Example usage
s, t = "abdcdbc", "abc"
print(len(s) - sum(x == y for x, y in zip_longest(s, t)))

Output:

4

The one-liner code leverages zip_longest from the itertools library to align characters from s and t. The comprehension then counts matching pairs, and the total length is adjusted accordingly.

## Summary/Discussion

• Method 1: Dynamic Programming. Offers optimal solution with precise subproblem optimization. The approach can be memory intensive for large sequences.
• Method 2: Two-Pointer Technique. Provides a clear and efficient way to solve the problem with a straightforward implementation. However, may not be as intuitive for those unfamiliar with the technique.
• Method 3: Recursion with Memoization. Good for demonstrating the concept of eliminating redundant calculations, but can be slower and more complex to understand compared to iterative solutions.
• Method 4: Greedy Algorithm. The simplest conceptually and fairly efficient for many cases, but not guaranteed to always provide the optimal solution.
• Bonus Method 5: List Comprehensions and itertools. Extremely concise and clever use of built-in functions, but potentially less efficient and harder to debug due to its compressed nature.