A sub-sequence of a given sequence is just the given sequence with some elements (possibly none or all) left out. We are given two sequences X[m] and Y[n] of lengths m and n respectively, with indexes of X and Y starting from 0. We wish to find the length of the longest common sub-sequence(LCS) of X[m] and Y[n] as l(m,n), where an incomplete recursive definition for the function l(i,j) to compute the length of The LCS of X[m] and Y[n] is given below:
l(i,j) = 0, if either i=0 or j=0
= expr1, if i,j > 0 and X[i-1] = Y[j-1]
= expr2, if i,j > 0 and X[i-1] != Y[j-1] (A)
expr1 ≡ l(i-1, j) + 1
(B)
expr1 ≡ l(i, j-1)
(C)
expr2 ≡ max(l(i-1, j), l(i, j-1))
(D)
expr2 ≡ max(l(i-1,j-1),l(i,j))
Answer: (C)
Explanation:
In Longest common subsequence problem, there are two cases for X[0..i] and Y[0..j]
1) The last characters of two strings match. The length of lcs is length of lcs of X[0..i-1] and Y[0..j-1] 2) The last characters don\'t match. The length of lcs is max of following two lcs values a) LCS of X[0..i-1] and Y[0..j] b) LCS of X[0..i] and Y[0..j-1]
Hence (C) is the correct answer.
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