Find all possible substrings after deleting k characters in Python

Sometimes we need to find all possible substrings after deleting exactly k characters from a string. This means keeping n-k characters in their original order to form new substrings. Python provides multiple approaches to solve this problem efficiently.

Using Loops and Recursion

This approach uses recursion to generate all combinations by selecting characters at different positions. We track the start and end positions while building each substring ?

def letterCombinations(s, temp, start, end, index, k):
    result = []
    
    if index == k:
        elem = ''.join(temp[:k])
        result.append(elem)
        return result
    
    for i in range(start, end - k + index + 2):
        temp[index] = s[i]
        result.extend(letterCombinations(s, temp, i + 1, end, index + 1, k))
    
    return result

stringA = 'Apple'
k = 1  # Delete 1 character, keep 4
temp = [''] * (len(stringA) - k)
start = 0
end = len(stringA) - 1

combinations = letterCombinations(stringA, temp, start, end, 0, len(stringA) - k)
result = set(combinations)
print(result)

{'Appe', 'Appl', 'Aple', 'pple'}

Using itertools.combinations

The itertools.combinations() function provides a cleaner approach by directly generating all possible character combinations of the desired length ?

from itertools import combinations

stringA = 'Apple'
k = 1  # Delete 1 character, keep 4

# Generate all combinations of (n-k) characters
result = set([''.join(combo) for combo in combinations(stringA, len(stringA) - k)])
print(result)

{'Appe', 'Appl', 'Aple', 'pple'}

Example with Multiple Deletions

Here's how to delete more characters from a longer string ?

from itertools import combinations

stringA = 'Python'
k = 2  # Delete 2 characters, keep 4

result = set([''.join(combo) for combo in combinations(stringA, len(stringA) - k)])
print(f"Original: {stringA}")
print(f"Substrings after deleting {k} characters: {result}")
print(f"Total combinations: {len(result)}")

Original: Python
Substrings after deleting 2 characters: {'Pyhn', 'Pthn', 'Pyon', 'ython', 'Phon', 'Pthon', 'yhon', 'Pyton', 'Pyto', 'thon', 'ythn', 'Pton', 'Pyho', 'ytho', 'Ptho'}
Total combinations: 15

Comparison

Conclusion

Use itertools.combinations() for clean, readable code when finding substrings after deleting k characters. The recursive approach helps understand the underlying logic but is more verbose.