LeetCode

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 isSymmetric(TreeNode *root) {
        if (!root || (!root->left && !root->right))
            return true;
        return dfs(root->left, root->right);
    }
    
    bool dfs(TreeNode *left, TreeNode *right)
    {
        if (!left && !right)
            return true;
        if ((!left && right) || (left && !right) || (left->val != right->val)) 
            return false;
        return dfs(left->left, right->right) && dfs(left->right, right->left);
    }
};