5 Best Ways to Find Length of Longest Consecutively Increasing Substring in Python

Rate this post

πŸ’‘ Problem Formulation: The challenge is to create a program that can scan through a given string and compute the length of the longest substring where characters are in consecutively increasing order. As an example, given the input “abcdab”, the longest consecutively increasing substring is “abcd”, yielding an output length of 4.

Method 1: Iterative Comparison

This method relies on a straightforward iteration over the input string, comparing adjacent characters to identify increments. Once a non-increasing character is encountered, it resets the count. The function maintains the maximum length observed during the iteration.

Here’s an example:

def longest_increasing_substring_length(s):
    max_length = 1
    current_length = 1
    for i in range(1, len(s)):
        if s[i] > s[i-1]:
            current_length += 1
            max_length = max(max_length, current_length)
            current_length = 1
    return max_length


Output: 4

This simple iterative function steps through the string, continuously counts increasing occurrences and resets when a decrease is found. Its time complexity is O(n), making it efficient for most use-cases.

Method 2: Dynamic Programming

Dynamic Programming can optimize the process by storing intermediate results. This example demonstrates how utilizing a list could help track the length of increasing substrings, updating the max length when necessary.

Here’s an example:

def longest_increasing_substring_length_dp(s):
    n = len(s)
    dp = [1] * n
    max_length = 1
    for i in range(1, n):
        if s[i] > s[i-1]:
            dp[i] = dp[i-1] + 1
            max_length = max(max_length, dp[i])
    return max_length


Output: 4

While the base case of this dynamic programming approach is similar to iterative comparison, it allows for more complex variations and extensions. The space complexity increases to O(n), but the time complexity remains O(n).

Method 3: Using GroupBy from itertools

This method leverages Python’s itertools.groupby() function to automatically group increasing sequences and find their lengths. It’s a concise and functional approach, ideal for Pythonic code enthusiasts.

Here’s an example:

import itertools

def longest_increasing_substring_length_itertools(s):
    max_length = 0
    for key, group in itertools.groupby(enumerate(s), lambda i_x: i_x[0]-ord(i_x[1])):
        group_length = len(list(group))
        max_length = max(max_length, group_length)
    return max_length


Output: 4

This code leverages a smart grouping based on the ordinal difference of characters and their indices, automatically segmenting the string and finding the maximum length.

Method 4: Regular Expressions

For those familiar with regular expressions, this method searches for all consecutively increasing substrings using a regex pattern and returns the length of the longest one found.

Here’s an example:

import re

def longest_increasing_substring_length_regex(s):
    increasing_substrings = re.findall(r'(?=((\w)(?=\w\2)))', s)
    max_length = max((len(m[0]) for m in increasing_substrings), default=0)
    return max_length + 1


Output: 4

This method uses regular expression to find all potential increasing pairs, computes their lengths, and determines the maximum. It’s a less intuitive approach but can be quite powerful in the hands of someone skilled in regex.

Bonus One-Liner Method 5: Functional Approach with reduce

A functional programming one-liner makes use of functools.reduce() to streamline the process, directly yielding the length of the longest increasing substring in a single line of code.

Here’s an example:

from functools import reduce

result = reduce(lambda r, a: (a, r[1]+1) if ord(a) == ord(r[0])+1 else ('', 1), "abcdab", ('', 0))[1]

Output: 4

The reduce() function aggregates character comparisons in an elegant, albeit dense, manner, using a lambda function. It is slick but may be difficult for those not used to functional programming paradigms.


  • Method 1: Iterative Comparison. Straightforward and efficient for most cases. May lack elegance in more complex scenarios.
  • Method 2: Dynamic Programming. Provides a good foundation for more complex operations and is both readable and maintainable. Uses extra space.
  • Method 3: Using GroupBy from itertools. Pythonic and concise. May not perform as well on very long strings due to list creation overhead.
  • Method 4: Regular Expressions. Powerful and compact, but potentially less readable to those unfamiliar with regex.
  • Method 5: Functional Approach with reduce. Extremely concise but may be confusing to less experienced programmers or those uncomfortable with functional programming.