Thomas Ngo

Software Engineer

3 minute read

Problem Statement

Given two strings text1 and text2, return the length of their longest common subsequence. If there is no common subsequence, return 0.

A subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.

For example, "ace" is a subsequence of "abcde".
A common subsequence of two strings is a subsequence that is common to both strings.

 

Example 1:

Input: text1 = "abcde", text2 = "ace" 
Output: 3  
Explanation: The longest common subsequence is "ace" and its length is 3.
Example 2:

Input: text1 = "abc", text2 = "abc"
Output: 3
Explanation: The longest common subsequence is "abc" and its length is 3.
Example 3:

Input: text1 = "abc", text2 = "def"
Output: 0
Explanation: There is no such common subsequence, so the result is 0.
 

Constraints:

1 <= text1.length, text2.length <= 1000
text1 and text2 consist of only lowercase English characters.

Intuition

My initial approach involves using recursion to explore all possible combinations and generate the common subsequence. Once a subsequence is found, I keep track of its length and compare it to the final result. Drawing from my experience with dynamic programming problems, I tend to start with a brute force approach and then intuitively improve it using memoization (top-down approach). Finally, I attempted to use memoization approach to produce the induction rules to build the dynamic programming approach from bottom-up.

Approach

I’m using a recursive approach with memoization to solve this problem. The idea is to compare characters at corresponding positions in both strings. If they match, I increment the count of the common subsequence and move on to the next characters. If they don’t match, I explore two possibilities: moving ahead in text1 and staying at the current position in text2, or vice versa. The maximum count from these possibilities is stored in a memoization table to avoid redundant calculations.

I’m also handling the case where the lengths of text1 and text2 are different by making sure that text1 is always the longer string.

Complexity

  • Time complexity: The time complexity is O(m * n), where m and n are the lengths of text1 and text2, respectively. This is because, in the worst case, we may have to explore all possible combinations of characters in both strings.

  • Space complexity: The space complexity is also O(m * n) due to the memoization table, which stores the results of subproblems to avoid redundant calculations.

Code - Memoization Approach

class Solution:
    def longestCommonSubsequence(self, text1: str, text2: str) -> int:
        if len(text1) < len(text2):
            return self.longestCommonSubsequence(text2, text1)

        t1 = text1
        t2 = text2
        memo = {}
        def dfs(t1_idx, t2_idx):
            if t1_idx == len(text1):
                return 0
            if t2_idx == len(text2):
                return 0

            if (t1_idx, t2_idx) in memo:
                return memo[(t1_idx, t2_idx)]

            if t1[t1_idx] == t2[t2_idx]:
                return dfs(t1_idx + 1, t2_idx + 1) + 1
            
            result = max(dfs(t1_idx + 1, t2_idx), dfs(t1_idx, t2_idx + 1))
            memo[(t1_idx, t2_idx)] = result
            return result

        return dfs(0, 0)

Editorial Solution

Dynamic Programming

The following notes are from Leet Code Editorial section. I used this to help me understand the problem solved by DP.

notes

notes-1

example: dp-note

Putting this all together, we iterate over each column in reverse, starting from the last column (we could also do rows, the final result will be the same). For a cell (row, col), we look at whether or not text1.charAt(row) == text2.charAt(col) is true. if it is, then we set grid[row][col] = 1 + grid[row + 1][col + 1]. Otherwise, we set grid[row][col] = max(grid[row + 1][col], grid[row][col + 1]).

For ease of implementation, we add an extra row of zeroes at the bottom, and an extra column of zeroes to the right.

Code

class Solution:
    def longestCommonSubsequence(self, text1: str, text2: str) -> int:
        
        # Make a grid of 0's with len(text2) + 1 columns 
        # and len(text1) + 1 rows.
        dp_grid = [[0] * (len(text2) + 1) for _ in range(len(text1) + 1)]
        
        # Iterate up each column, starting from the last one.
        for col in reversed(range(len(text2))):
            for row in reversed(range(len(text1))):
                # If the corresponding characters for this cell are the same...
                if text2[col] == text1[row]:
                    dp_grid[row][col] = 1 + dp_grid[row + 1][col + 1]
                # Otherwise they must be different...
                else:
                    dp_grid[row][col] = max(dp_grid[row + 1][col], dp_grid[row][col + 1])
        
        # The original problem's answer is in dp_grid[0][0]. Return it.
        return dp_grid[0][0]

Time Complexity: O(m * n) since we solved m * n subproblems Space Complexity: O(m * n)

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