【Leetcode】Symmetric Tree

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

分析：前面提过二叉树的题目主要都可以用递归和遍历解决。这题也能用递归进行解决。

/**
 * 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 == nullptr)
            return true;
        
        return judgeSymmetric(root->left, root->right);
    }
    
private:
    bool judgeSymmetric(TreeNode *node, TreeNode *mirror)
    {
        if(node == nullptr && mirror == nullptr)
            return true;
        else if(node != nullptr && mirror != nullptr && node->val == mirror->val)
            return judgeSymmetric(node->right, mirror->left) && judgeSymmetric(node->left, mirror->right);
        else
            return false;
    }
};