101.symmetric-tree.java

/*
 * @lc app=leetcode id=101 lang=java
 *
 * [101] Symmetric Tree
 */

// @lc code=start
/**
 * 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;
 *     }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        return helper(root, root);
    }

    public boolean helper(TreeNode node1, TreeNode node2) {
        if (node1 == null && node2 == null) {
            return true;
        }
        if (node1 == null || node2 == null) {
            return false;
        } 

        return node1.val == node2.val && helper(node1.left, node2.right) && helper(node1.right, node2.left);
    }
}
// @lc code=end