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This is a medium difficulty Coding & Algorithms question, commonly asked during Onsite rounds at TikTok.

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This question is commonly asked for Software Engineer candidates at TikTok during technical interviews.

Find longest common subsequence length

Quick Overview

This question evaluates understanding of dynamic programming and sequence comparison algorithms by asking for the length of the longest common subsequence between two strings.

Find longest common subsequence length

Company: TikTok

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Onsite

Given two strings `s` and `t`, compute the length of their **longest common subsequence** (LCS). A subsequence is obtained by deleting zero or more characters from a string without changing the relative order of the remaining characters. ## Input - Two strings `s` and `t` (may be empty) ## Output - An integer: the maximum length of any common subsequence of `s` and `t` ## Example - `s = "abcde"`, `t = "ace"` → output `3` ("ace") ## Constraints (you may assume) - `0 <= len(s), len(t) <= 2000` - Characters are ASCII letters (or general bytes).

Quick Answer: This question evaluates understanding of dynamic programming and sequence comparison algorithms by asking for the length of the longest common subsequence between two strings.

Given two strings s and t, return the length of their longest common subsequence (LCS). A subsequence is formed by deleting zero or more characters from a string without changing the relative order of the remaining characters. Note that a subsequence does not need to be contiguous. For example, in the strings "abcde" and "ace", the longest common subsequence is "ace", so the answer is 3.

Constraints

0 <= len(s), len(t) <= 2000
Strings may be empty
Characters are ASCII letters or general bytes
An O(len(s) * len(t)) dynamic programming solution is acceptable

Examples

Input: ("abcde", "ace")

Expected Output: 3

Explanation: "ace" is a common subsequence of both strings, and no longer common subsequence exists.

Input: ("abc", "abc")

Expected Output: 3

Explanation: The entire string matches, so the LCS length is 3.

Input: ("abc", "def")

Expected Output: 0

Explanation: The two strings share no common characters, so the longest common subsequence has length 0.

Input: ('', 'abc')

Expected Output: 0

Explanation: If one string is empty, there cannot be any common subsequence other than the empty one.

Input: ("aggtab", "gxtxayb")

Expected Output: 4

Explanation: One longest common subsequence is "gtab", which has length 4.

Input: ("aaaa", "aa")

Expected Output: 2

Explanation: Repeated characters still count in order; the longest common subsequence is "aa".

Hints

Think about prefixes: what is the answer for s[:i] and t[:j] based on smaller prefixes?
If the current characters match, you can extend a smaller subsequence; otherwise, you may need to skip one character from either string.

Last updated: May 11, 2026

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