day45（12.26）——leetcode面试经典150

101. 对称二叉树

101. 对称二叉树

主播写的很有成就感！！！

题目：

题解：

/**
 * 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 dfs(TreeNode left, TreeNode right) {
        if(left == null && right == null) {
            return true;
        } 
        else if(left == null || right == null) {
            return false;
        }
        else if(left.val != right.val) {
            return false;
        }
        else if(dfs(left.left, right.right) == false) {
            return false;
        }
        else if(dfs(left.right, right.left) == false) {
            return false;
        } 
        return true;
    }

    public boolean isSymmetric(TreeNode root) {
        if(root == null) {
            return true;
        }
        return dfs(root.left, root.right);

    }
}