Given two string X and Y. The task is to find the length of longest subsequence of string X which is substring in sequence Y.
Input : X = "ABCD", Y = "BACDBDCD" Output : 3 "ACD" is longest subsequence of X which is substring of Y. Input : X = "A", Y = "A" Output : 1
Method 1 (Brute Force):
Use brute force to find all the subsequence of X and for each subsequence check whether it is substring of Y or not. If it is substring of Y, maintain a maximum length varible and compare length with it.
Method 2: (Dynamic Programming):
Let n be length of X and m be length of Y. Create a 2D array ‘dp’ of m + 1 rows and n + 1 columns. Value dp[i][j] is maximum length of subsequence of X[0….j] which is substring of Y[0….i]. Now for each cell of dp fill value as :
for (i = 1 to m) for (j = 1 to n) if (x[i-1] == y[j - 1]) dp[i][j] = dp[i-1][j-1] + 1; else dp[i][j] = dp[i][j-1];
And finally, the length of the longest subsequence of x which is substring of y is max(dp[i][n]) where 1 <= i <= m.
Below is implementation this approach:
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