LeetCode（101）Symmetric Tree

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.

分析

判断一棵二叉树是否为对称树；

仍然采用递归的思想，判断该树的左右子树是否对称；

若二叉树p与二叉树q对称，也就是说其根节点相同，p左子树应与q右子树对称，同理，p右子树应与q左子树对称；

AC代码

/**
 * Definition for a binary tree node.
 * 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)
            return true;
        else
            //判断左右子树是否对称
            return isSymmetricTree(root->left, root->right);
    }

    bool isSymmetricTree(TreeNode* p, TreeNode* q) {
        //如果两个二叉树均为空，则返回true
        if (!p && !q)
        {
            return true;
        }
        //如果两者其一为空树，则返回false
        else if (!p || !q)
        {
            return false;
        }
        else{
            if (p->val != q->val)
                return false;
            else
                //p左子树应与q右子树对称，同理，p右子树应与q左子树对称
                return isSymmetricTree(p->left, q->right) && isSymmetricTree(p->right, q->left);
        }
    }
};