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

题目：给定一个二叉树，每个节点都是由1~9的数字组成，问能组成伪回文的线路有几条。

思路：由于节点是由1~9的数字组成，用一个长度为10的vector保存每条路个数字出现的次数。如果出现次数为奇数次的数字数量小于等于 1，则证明该条路可以组成伪回文，结果加1. 代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void helper(TreeNode* node, vector<int>& record, int& res){
        if(!node) return;
        record[node->val]++;
        if(!node->left && !node->right){
            int odd = 0;
            for(int i = 1; i < 10; i++){
                if(record[i] % 2 == 1) odd++;
            }
            if(odd <= 1) res++;
        }
        if(node->left) helper(node->left, record, res);
        if(node->right) helper(node->right,record, res);
        record[node->val]--;
    }
    int pseudoPalindromicPaths (TreeNode* root) {
        int res = 0;
        vector<int> record(10, 0);
        helper(root, record, res);
        return res;
    }
};