How to write a function x = ubidisol(u,f,b) to solve the involved upper bidiagonal system

A and b are,

A = diag(2*ones(1,n)) - diag(ones(1,n-1),1) - diag(ones(1,n-1),-1) b = [0:1:n/2-1 n/2-1:-1:0]’

LU decomposition of A is already found by different function,to solve the five linear systems of (b) by using lbidisol.m and ubidisol.m without re-decomposing the matrix A (lbidisol.m is also written). how do we write the function x = ubidisol(u,f,b) to solve the involved upper bidiagonal system in order to solve the system A^5x = b.