Leetcode 树 Symmetric Tree

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Symmetric Tree

  Total Accepted: 13991  Total Submissions: 44240

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

Note:
Bonus points if you could solve it both recursively and iteratively.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

题意：给定一棵二叉树，判断它是否对称
思路：dfs递归

一棵二叉树对称的条件：
1.左右子树的根节点一样
2.左子树的左子树和右子树的右子树对称
3.左子树的右子树和右子树的左子树对称
复杂度：时间O(n) ，空间O(log n)
相关题目：Same Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode *left, TreeNode *right)
    {
        if(!left && !right ) return true;
    	if(!left ||  !right) return false;
    	if(left->val != right->val) return false;
    	return (isSymmetric(left->left, right->right) && isSymmetric(left->right, right->left));
    
    }
    bool isSymmetric(TreeNode *root){
    	if(!root) return true;
    	return isSymmetric(root->left, root->right);
    }
};