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mediumDSA

How do you find the longest common subsequence?

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01

Understand the problem

Explain LCS.

dplcs
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the solution

Use a 2D DP table where dp[i][j] is the LCS length of prefixes. If characters match, dp[i][j] = dp[i-1][j-1]+1; else the max of dropping one character from either string. O(m×n).

Solution ready — 2 min read

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04

Read the code

LCS length (2-D DP)
Run Playground
def lcs(a, b):
    m, n = len(a), len(b)
    dp = [[0] * (n + 1) for _ in range(m + 1)]
    for i in range(1, m + 1):
        for j in range(1, n + 1):
            if a[i-1] == b[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[m][n]


# --- demo ---
print(lcs("ABCBDAB", "BDCAB"))   # 4  (e.g. "BCAB")
05

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