1457. Pseudo-Palindromic Paths in a Binary Tree

How to solve Pseudo-Palindromic Paths in a Binary Tree (LeetCode 1457): the approach in brief

The problem

Given a binary tree where node values are digits from 1 to 9. A path in the binary tree is said to be pseudo-palindromic if at least one permutation of the node values in the path is a palindrome.

Pattern

Pseudo-Palindromic Paths in a Binary Tree is a Backtracking Patterns problem, in the Palindrome Partitioning subpattern. The pattern covers subsets, permutations, combinations, N-Queens, and word search

Partition string into palindromic substrings using backtracking.

Approach outline

1. Consider using a depth-first traversal technique to explore all root-to-leaf paths in the binary tree.
2. As you traverse, think about how to efficiently track the frequency of node values along the current path without storing the entire path.
3. Recall that a palindrome can have at most one character with an odd frequency count; how can this property help in checking pseudo-palindromic paths?

Complexity

The reference solution runs in O(n) time and O(h) space.

Asked at

Pseudo-Palindromic Paths in a Binary Tree has been reported in interviews at Amazon.