How to get upper triangluar matrix from lower triangular matrix?

If L'L = U'U, where L and U are lower and upper triangular matrices respectively, then how to get U directly from L?

P.S.: I already knew one solution is U = QR (L), but this operation need O(N^3) if L's size is N*N. I am wondering whether there is a O(N^2) operation for solving triangular matrix.

Answer 1

[Veera Kanmani]

https://in.mathworks.com/matlabcentral/answers/152818-how-to-covert-a-vector-of-lower-triangular-matrix-factors-back-to-the-original-matrix