【leetcode】Symmetric Tree

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

---

题解：递归的解法，重新写一个递归函数bool issysmem(TreeNode *lc,TreeNode &rc)，函数输入分别输入待比较是否对称的两个树的树根。首先比较这两棵树根的值是否相等，然后递归的比较lc的左子和rc的右子，以及lc的右子和rc的左子是否对称。代码如下:

 1 /**

 2  * Definition for binary tree

 3  * struct TreeNode {

 4  *     int val;

 5  *     TreeNode *left;

 6  *     TreeNode *right;

 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 8  * };

 9  */

10 class Solution {

11 public:

12     bool isSymmetric(TreeNode *root) {

13         if(root == NULL)

14             return true;

15         return issysmem(root->left,root->right);

16     }

17     bool issysmem(TreeNode *lc,TreeNode *rc){

18         if((!lc && rc) || (lc && !rc))

19             return false;

20         if(lc == NULL && rc == NULL)

21             return true;

22         if(lc->val != rc->val)

23             return false;

24         return issysmem(lc->left,rc->right) && issysmem(lc->right,rc->left);

25     }

26 };