从零开始的LC刷题(22): Symmetric Tree 二叉树是否对称

原题：

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

和上一道题一样，要中序遍历dfs，不过右子树要反向dfs，结果如下：

Success

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Symmetric Tree.

Memory Usage: 14.7 MB, less than 85.91% of C++ online submissions for Symmetric Tree.

代码：

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(root==NULL){return true;}
        return Compare(root->left,root->right);
    }
    
    bool Compare(TreeNode* l,TreeNode* r){
        if(l==NULL&&r==NULL){return true;}
        if(l==NULL||r==NULL){return false;}
        if(Compare(l->left,r->right)==true){
            if(l->val==r->val){
                if(Compare(l->right,r->left)==true){
                    return true;
                }
            }
        }
        return false;
    }
    
};