leetcode 第101题 Symmetric tree

最新推荐文章于 2021-12-25 20:24:37 发布

zhang_shuai12

最新推荐文章于 2021-12-25 20:24:37 发布

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分类专栏： leetcode题解文章标签：对称树-递归

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本文介绍如何通过递归实现判断二叉树是否对称的算法，包括C++代码实现，涉及序列化和反序列化二叉树的概念。

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Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

/ \
2 2
/ \ / \
3 4 4 3

But the following is not:

/ \
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}”.

思路：递归实现，自顶向下比较。
c++实现：

/**
 * 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 checknode(TreeNode *leftnode,TreeNode *rightnode){
        if(leftnode == NULL && rightnode == NULL)
            return true;
        if(leftnode == NULL || rightnode == NULL)
            return false;
        return leftnode->val == rightnode->val && checknode(leftnode->left,rightnode->right) && checknode(leftnode->right,rightnode->left);
    }
    bool isSymmetric(TreeNode *root) {
        if(root == NULL)
            return true;
        return checknode(root->left,root->right);
    }
};