【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:

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