Is Count Substrings That Differ by One Character easy or hard?

The problem is rated Medium because the brute force idea is straightforward but inefficient. The challenge is recognizing the diagonal traversal and sliding window technique that avoids repeated substring comparisons. Once the mismatch-tracking idea is clear, the implementation becomes manageable.

Count Substrings That Differ by One Character Python/Java solution

Both Python and Java implementations follow the same logic: iterate over all diagonal alignments between the two strings and maintain a window that allows exactly one mismatch. Track the last mismatch index and update the number of valid substrings ending at each position. The complexity remains O(n * m) with constant extra space.

How to solve Count Substrings That Differ by One Character in O(n*m)?

Align the two strings along diagonals and iterate through each alignment. Maintain a sliding window that allows exactly one mismatch by storing the last mismatch position. When a second mismatch occurs, shift the window start to maintain the single-mismatch condition. This technique processes each aligned pair once, giving O(n * m) time.

What is the best approach for Count Substrings That Differ by One Character?

The most efficient approach uses diagonal traversal with a sliding window that tracks mismatches between aligned characters of the two strings. For each alignment of s and t, expand a window while recording the last mismatch position. When a second mismatch appears, move the window start forward. This counts valid substrings in O(n * m) time with O(1) extra space.

Is Count Substrings That Differ by One Character asked at Google/Amazon/Meta?

String comparison and substring enumeration problems like this appear frequently in interviews at large tech companies. Variants that involve counting mismatches, sliding windows, or dynamic programming over strings have been reported in interviews at companies such as Amazon, Google, and Meta.

What data structure is used in Count Substrings That Differ by One Character?

The optimized solution mainly relies on string traversal with two-pointer or sliding window logic. No heavy data structures are required; the algorithm tracks mismatch indices while scanning aligned characters. Some explanations frame the idea using dynamic programming concepts for counting valid substring ranges.

What is the time complexity of Count Substrings That Differ by One Character?

The optimized solution runs in O(n * m) time, where n and m are the lengths of the two strings. Each diagonal alignment of the strings is processed once using a sliding window. The brute force substring comparison approach can reach O(n^3) due to repeated character comparisons.

Description Hints 2 Solution Video 10:20

1638. Count Substrings That Differ by One Character

Medium72.4% acceptance1 company

Given two strings s and t, find the number of ways you can choose a non-empty substring of s and replace a single character by a different character such that the resulting substring is a substring of t. In other words, find the number of substrings in s that differ from some substring in t by exactly one character.

For example, the underlined substrings in "computer" and "computation" only differ by the 'e'/'a', so this is a valid way.

Return the number of substrings that satisfy the condition above.

A substring is a contiguous sequence of characters within a string.

Example 1:

Input: s = "aba", t = "baba"
Output: 6
Explanation: The following are the pairs of substrings from s and t that differ by exactly 1 character:
("aba", "baba")
("aba", "baba")
("aba", "baba")
("aba", "baba")
("aba", "baba")
("aba", "baba")
The underlined portions are the substrings that are chosen from s and t.

Example 2:

Input: s = "ab", t = "bb"
Output: 3
Explanation: The following are the pairs of substrings from s and t that differ by 1 character:
("ab", "bb")
("ab", "bb")
("ab", "bb")
The underlined portions are the substrings that are chosen from s and t.

Constraints:

1 <= s.length, t.length <= 100
s and t consist of lowercase English letters only.

Asked at 1 company

Microsoft

Topics

Hints open one at a time

Hint 1

Take every substring of s, change a character, and see how many substrings of t match that substring.

Hint 2

Use a Trie to store all substrings of t as a dictionary.

10:20

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