6. Longest Common Subsequence

The problem can be found at the following link: Question Link

Problem Description

Given two strings s1 and s2, return the length of their longest common subsequence (LCS). If there is no common subsequence, return 0.

A subsequence is a sequence that can be derived from the given string by deleting some or no elements without changing the order of the remaining elements. For example, "ABE" is a subsequence of "ABCDE".

Examples

Example 1:

Input:

s1 = "ABCDGH", s2 = "AEDFHR"

Output:

3

Explanation:

The longest common subsequence of "ABCDGH" and "AEDFHR" is "ADH", which has a length of 3.

Example 2:

Input:

s1 = "ABC", s2 = "AC"

Output:

2

Explanation:

The longest common subsequence of "ABC" and "AC" is "AC", which has a length of 2.

Example 3:

Input:

s1 = "XYZW", s2 = "XYWZ"

Output:

3

Explanation:

The longest common subsequences of "XYZW" and "XYWZ" are "XYZ" and "XYW", both of length 3.

Constraints:

• $(1 \leq \text{length}(s1), \text{length}(s2) \leq 10^3)$

• Both s1 and s2 contain only uppercase English letters.

My Approach

Space-Optimized 1D DP

Key Idea

• Use only two arrays (current and previous rows) to store LCS lengths for substrings.

• The value at dp[j] represents the LCS length of s1[0..i-1] and s2[0..j-1].

• Only the previous row is needed to compute the current row, so space is reduced to O(M), where M is the length of the second string.

Algorithm Steps

1. Use two rolling 1D arrays (or one array with a few variables) to store only the previous row and current row.

2. Traverse the strings from i = 1..n and j = 1..m:

   • If s1[i - 1] == s2[j - 1], we do curr[j] = prev[j - 1] + 1.

   • Else, curr[j] = max(prev[j], curr[j - 1]).

3. Swap prev and curr at the end of each i iteration.

4. The result is prev[m], which is the length of LCS.

This approach is more space-efficient than the standard O(N * M) DP table because it only keeps two rows (or one row with updates).

Time and Auxiliary Space Complexity

• Expected Time Complexity: O(N * M), where N is the length of s1 and M is the length of s2.

• Expected Auxiliary Space Complexity: O(N), using the space-optimized approach (storing only one dimension).

Code (C++)

class Solution {
public:
    int lcs(string &s1, string &s2) {
        int n = s1.size(), m = s2.size();
        vector<int> prev(m + 1, 0), curr(m + 1, 0);
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= m; j++) {
                curr[j] = s1[i - 1] == s2[j - 1] ? prev[j - 1] + 1 : max(prev[j], curr[j - 1]);
            }
            swap(prev, curr);
        }
        return prev[m];
    }
};

⚡ Alternative Approaches

2️⃣ Dynamic Programming with 2D Table (O(N _ M) Time, O(N _ M) Space)

Algorithm Steps:

1. Create a 2D table dp where dp[i][j] stores the length of the LCS of s1[0..i-1] and s2[0..j-1].

2. If characters match, dp[i][j] = dp[i-1][j-1] + 1.

3. If characters don't match, dp[i][j] = max(dp[i-1][j], dp[i][j-1]).

4. Return dp[n][m], the LCS length.

class Solution {
public:
    int lcs(string &s1, string &s2) {
        int n = s1.size(), m = s2.size();
        vector<vector<int>> dp(n + 1, vector<int>(m + 1, 0));
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= m; j++) {
                if (s1[i - 1] == s2[j - 1]) dp[i][j] = dp[i - 1][j - 1] + 1;
                else dp[i][j] = max(dp[i - 1][j], dp[i][j - 1]);
            }
        }
        return dp[n][m];
    }
};

🔹 Easier to visualize the DP table. 🔹 Simpler to reconstruct the actual LCS if needed.

3️⃣ Recursive + Memoization (Top-Down DP, O(N * M))

Algorithm Steps:

1. Define a function helper(i, j) that returns the LCS of s1[0..i-1] and s2[0..j-1].

2. If characters match, 1 + helper(i-1, j-1).

3. Otherwise, max(helper(i-1, j), helper(i, j-1)).

4. Use a 2D memo array dp[i][j] to avoid recalculations.

class Solution {
public:
    int lcs(string &s1, string &s2) {
        int n = s1.size(), m = s2.size();
        vector<vector<int>> dp(n + 1, vector<int>(m + 1, -1));
        return helper(s1, s2, n, m, dp);
    }
    int helper(string &s1, string &s2, int i, int j, vector<vector<int>> &dp) {
        if (i == 0 || j == 0) return 0;
        if (dp[i][j] != -1) return dp[i][j];
        if (s1[i - 1] == s2[j - 1])
            return dp[i][j] = 1 + helper(s1, s2, i - 1, j - 1, dp);
        return dp[i][j] = max(helper(s1, s2, i - 1, j, dp), helper(s1, s2, i, j - 1, dp));
    }
};

🔹 Top-down approach can be more intuitive for some. 🔹 Same O(N*M) complexity with memoization.

📊 Comparison of Approaches

Approach	⏱️ Time Complexity	🗂️ Space Complexity	✅ Pros	⚠️ Cons
Space-Optimized 1D DP	🟢 O(N * M)	🟢 O(N)	Memory efficient	No reconstruction
2D Dynamic Programming	🟢 O(N * M)	🔴 O(N * M)	Full table stored	Higher space usage
Memoized Recursive	🟢 O(N * M)	🟡 O(N * M)	Intuitive for some	Needs recursion overhead

💡 Best Choice?

• ✅ Use Space-Optimized 1D DP for length-only queries.

• ✅ Use 2D DP with Backtracking if you need to recover the actual LCS string.

• ✅ Use Recursive + Memoization for simple coding competitions (top-down is intuitive for some coders).

Code (Java)

class Solution {
    static int lcs(String s1, String s2) {
        int n = s1.length(), m = s2.length();
        int[] prev = new int[m + 1], curr = new int[m + 1];
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= m; j++) {
                curr[j] = s1.charAt(i - 1) == s2.charAt(j - 1) ? prev[j - 1] + 1 : Math.max(prev[j], curr[j - 1]);
            }
            int[] temp = prev; prev = curr; curr = temp;
        }
        return prev[m];
    }
}

Code (Python)

class Solution:
    def lcs(self, s1, s2):
        m, n = len(s1), len(s2)
        dp = [0] * (n + 1)
        for i in range(m):
            prev = 0
            for j in range(n):
                temp = dp[j + 1]
                dp[j + 1] = prev + 1 if s1[i] == s2[j] else max(dp[j + 1], dp[j])
                prev = temp
        return dp[n]

Contribution and Support:

For discussions, questions, or doubts related to this solution, feel free to connect on LinkedIn: Any Questions. Let’s make this learning journey more collaborative!

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