4.4 leetcode -4 symmetric-tree

题目描述

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

Note:
Bonus points if you could solve it both recursively and iteratively.
confused what"{1,#,2,3}"means? > read more on how binary tree is serialized on OJ.

OJ's Binary Tree Serialization:
The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.
Here's an example:
1
/ \
2 3
/
4
\
5

The above binary tree is serialized as"{1,2,3,#,#,4,#,#,5}".

判断是否是镜像树，递归即可

/** * 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 == NULL)            return true;        else        return isEqual(root->left,root->right);    }    bool isEqual(TreeNode *left,TreeNode *right)        {        if((left== NULL && right == NULL))            return true;        else if(left== NULL && right != NULL)            return false;        else if(left!= NULL && right == NULL)            return false;        else            {                if(left->val == right->val)                    {                    return isEqual(left->left,right->right) && isEqual(left->right,right->left);                }                else                    {                    return false;                }        }    }};