Symmetric Tree

问题：判断二叉树是否为镜像二叉树
分析：递归判断，根节点单独判断，然后递归左结点和右结点，之后每次一起递归左结点的左结点和右结点的右结点比较，左结点的右结点和右结点的左结点比较

/**

 * 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 dfs(TreeNode *left,TreeNode *right)

    {

        if(!left && !right) return true;//需要先判断这个，不然val那个可能会出现RE

        if((left && !right) || (!left && right) || (left->val!=right->val)) return false;

        return dfs(left->left,right->right) && dfs(left->right,right->left);

    }

    bool isSymmetric(TreeNode *root) {

        if(root==NULL || (!root->left && !root->right)) return true;

        return dfs(root->left,root->right);

    }

};