LC.P1457[二叉树中的伪回文路径]

方法一：DFS+位运算

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public int pseudoPalindromicPaths(TreeNode root) {
        return dfs(root, 0);
    }

    private int dfs(TreeNode root, int mask) {
        if (root == null) return 0;
        mask ^= 1 << root.val; // 修改 root.val 出现次数的奇偶性
        if (root.left == root.right) {
            return (mask & (mask - 1)) == 0 ? 1 : 0;
        }
        return dfs(root.left, mask) + dfs(root.right, mask);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$