 # check existence of a matrix in another matrix

if there is a matrix A[][] of order m and another matrix B[][] of order n such that (m>n) you have to find the occurrence of matrix B[][] in matrix A[][].

``````A=1,2,3,4,5
5,4,1,9,7
2,1,7,3,4
6,4,8,2,7
0,2,4,5,8

B=1,9,7
7,3,4
8,2,7
``````

This matrix B exist in A.
I can do it by sliding window algo TC O(p^2*n^2) where p = m-n+1. but I want to do this with minimum time complexity.

Baker-Bird algorithm is what you looking for. In a brief: you split matrix B on separate rows and obtain n independent patterns, you want to look in matrix A. Now you want for each position in A compute, whether there ends some row of matrix B. You can store it in some integer matrix C such as on position C[i][j] is number k if on position A[i][j] is k-th row of matrix B. This can be easily done with Aho-Corasick algorithm.

Now when you fill matrix C, problem is really simple. You just want to find column in this matrix, that there is sequence 1, 2, 3 … n. This means, that you find occurence of B in A. Total time complexity is O(n^2 + m^2) which is optimal.

But this is pretty nasty and I assume, that rows of B are distinct. If not, it’s still possible to solve, but in matrix C you don’t look for 1, 2 … n but something like 1, 2, 1… if first and third rows of B are equal and you must add KMP algorithm to it.