Leetcode 2791. Count Paths That Can Form a Palindrome in a Tree

Count pairs of nodes in a rooted tree whose path edge-letters can be rearranged into a palindrome — equivalently, the multiset of letters on the path has at most one character with odd count. This reduces to comparing parity bitmasks of root-to-node letter counts: pairs with equal masks or masks differing by one bit form valid paths.