Leetcode 101 Symmetric Tree

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

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

But the following [1,2,2,null,3,null,3] is not:

    1   / \  2   2   \   \   3    3

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

判断一棵二叉树是否对称。

稍微改变一下100题的思路，遍历的时候以不同方向遍历左右子树，变成判断根节点左右子树是否相同就行了

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool dfs(TreeNode* t1,TreeNode* t2)    {        if(!t1 || !t2)        {            if(!t1 && !t2) return true;            return false;        }        if(t1->val!=t2->val) return false;        return dfs(t1->left,t2->right) && dfs(t1->right,t2->left);    }    bool isSymmetric(TreeNode* root)     {        if(!root) return true;        return dfs(root->left,root->right);    }};