On page 84 of Russell & Norvig's book "Artificial Intelligence: A Modern Approach Book" (3rd edition), the pseudocode for uniform cost search is given. I provided a screenshot of it here for your convenience.
I am having trouble understanding the highlighted line
if child.STATE is not in explored **or** frontier then
Shouldn't that be
if child.STATE is not in explored **and** frontier then
The way it's written, it seems to suggest that if the child node has already been explored, but not currently in the frontier, then we add it to the frontier, but I don't think that's correct. If it's already been explored, then it means we already found the optimal path for this node previously and should not be processing it again. Is my understanding wrong?