leetcode第48题（symmetric-tree）

最新推荐文章于 2025-02-24 02:00:00 发布

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最新推荐文章于 2025-02-24 02:00:00 发布

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分类专栏：牛客网（leetcode）文章标签：对称性质

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本文介绍了一种检查二叉树是否为中心对称的方法，通过递归比较左子树和右子树来判断整棵树是否对称。文章提供了一个具体的实现示例，并解释了如何进行二叉树的序列化。

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题目：

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

For example, this binary tree is symmetric:

But the following is not:

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:

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

思路：根据前序遍历（根、左、右）和对称的前序遍历（根、右、左）即可判断。只要左子树的左结点和右子树的右结点，左子树的右节点和右子树的左结点相等，就是对称的。

代码：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isSymmetrical(root.left, root.right);
    }

    public boolean isSymmetrical(TreeNode left, TreeNode right) {
        if (left == null && right == null) return true;
        if (left == null || right == null) return false;
        if (left.val == right.val)
            return isSymmetrical(left.left, right.right) && isSymmetrical(left.right, right.left);
        return false;
    }
}

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

For example, this binary tree is symmetric:

But the following is not:

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:

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