LeetCode:对称二叉树

题目链接：https://leetcode-cn.com/problems/symmetric-tree/description/

给定一个二叉树，检查它是否是镜像对称的。

思路：DFS 深度搜索 左节点和右节点  右节点和左节点比较

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution { 
    public boolean issymmetric=true;
    public void isTreeSymmetric(TreeNode leftNode,TreeNode rightNode){
        if(leftNode==null&&rightNode==null){
            return ;
        }
        if(leftNode==null){
            issymmetric=false;
            return ;
        }
        if(rightNode==null){
            issymmetric=false;
            return ;
        }
        if(leftNode.val!=rightNode.val){
            issymmetric=false;
            return ;
        }
        isTreeSymmetric(leftNode.left,rightNode.right);
        isTreeSymmetric(leftNode.right,rightNode.left);
    }
    public boolean isSymmetric(TreeNode root) {
        if(root==null){
            return true;
        }
        isTreeSymmetric(root.left,root.right);
        return issymmetric;
    }
}