1457. Pseudo-Palindromic Paths in a Binary Tree

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.

Return the number of pseudo-palindromic paths going from the root node to leaf nodes.

Example 1:

Input: root = [2,3,1,3,1,null,1]
Output: 2 
Explanation: The figure above represents the given binary tree. There are three paths going from the root node to leaf nodes: the red path [2,3,3], the green path [2,1,1], and the path [2,3,1]. Among these paths only red path and green path are pseudo-palindromic paths since the red path [2,3,3] can be rearranged in [3,2,3] (palindrome) and the green path [2,1,1] can be rearranged in [1,2,1] (palindrome).

Example 2:

Input: root = [2,1,1,1,3,null,null,null,null,null,1]
Output: 1 
Explanation: The figure above represents the given binary tree. There are three paths going from the root node to leaf nodes: the green path [2,1,1], the path [2,1,3,1], and the path [2,1]. Among these paths only the green path is pseudo-palindromic since [2,1,1] can be rearranged in [1,2,1] (palindrome).

Example 3:

Input: root = [9]
Output: 1

Constraints:

• The given binary tree will have between 1 and 10^5 nodes.
• Node values are digits from 1 to 9.

class Solution {
    public int pseudoPalindromicPaths (TreeNode root) {
        return search(root, new HashSet<>());
    }
    public int search(TreeNode tn, HashSet<Integer> set) {
        if (tn == null) {
            return 0;
        }
        
        if (set.contains(tn.val)) {
            set.remove(tn.val);
        } else {
            set.add(tn.val);
        }
        
        int res = 0;
        if (tn.left == null && tn.right == null && set.size() <= 1) {
            res++;
        }
        
        res += search(tn.left, new HashSet(set)) + search(tn.right, new HashSet(set));
        return res;
    }
}

https://leetcode.com/problems/pseudo-palindromic-paths-in-a-binary-tree/discuss/648332/Java-O(N)-DFS-HashSet-Bitwise

class Solution {
    int count = 0;
    public int pseudoPalindromicPaths (TreeNode root) {
        dfs(root, new HashSet<>());
        return count;
    }
    private void dfs(TreeNode node, Set<Integer> set) {
        if(node !=null) {
            if(set.contains(node.val)) set.remove(node.val); else set.add(node.val);
            if(node.left ==null && node.right == null) {
                if(set.size() <=1) count++;
            } else {
                dfs(node.left, new HashSet<>(set));
                dfs(node.right, new HashSet<>(set));                
            }
        }
    }
}