How to Fix Longest Common Subsequence Errors

In this tutorial, you'll learn about How to Fix Longest Common Subsequence Errors. We cover key concepts, practical examples, and best practices.

Fix longest common subsequence errors when DP table initialization wrong or character direction reversed.

Quick Fix

Wrong

def lcs(a,b):
    n,m=len(a),len(b)
    dp=[[0]*m]*n  # wrong: copies row reference

Wrong initialization creates shared references. Updating one row affects all rows.

Right

def lcs(a,b):
    n,m=len(a),len(b)
    dp=[[0]*(m+1) for _ in range(n+1)]
    for i in range(1,n+1):
        for j in range(1,m+1):
            if a[i-1]==b[j-1]: dp[i][j]=1+dp[i-1][j-1]
            else: dp[i][j]=max(dp[i-1][j],dp[i][j-1])
    return dp[n][m]

lcs('abcde','ace')=3 ('ace'). O(n*m).

Prevention

Initialize (n+1)x(m+1) with 0. if match: dp[i][j]=1+dp[i-1][j-1]. Else: max(left, above).

DodaTech Tools

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