The rows and columns of the grid will be equal to the length of the given string s. Let's consider the given string s to be "cbbd". We will have a 4 x 4 grid in this case. Since we want to find the length of the longest palindromic subsequence, each cell in the grid will store the length of the longest palindromic subsequence encountered so far during the bottom-up traversal.

Now for the bottom-up traversal, two pointers are initialized at the start and end of the original string. Essentially, one pointer would traverse normally, while the other would work in reverse. This kind of traversal is required to check for palindromes (as they read the same forwards and backward) so that we only store the lengths of palindromic sequences in the grid.

1for i in range(len(s) - 1, -1, -1): # works in reverse
2    for j in range(i + 1, len(s)): # traverses normally