Symmetric Tree <LeetCode>

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


思路：首先把想到用递归分别遍历左右子树，不同的时，遍历左子树时，先遍历根节点，然后是左孩子，最后右孩子；遍历右子树时，县遍历根节点，然后是右孩子，最后是左孩子，每遍历到一个节点进行比较，是否相同，代码如下：//没有优化

/**
 * 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;
        TreeNode *rootl;
        TreeNode *rootr;
        rootl=root->left;
        rootr=root->right;
        return isSame(rootl,rootr);
    }
    
    bool isSame(TreeNode *rootl,TreeNode *rootr)
    {
        if(rootl==NULL||rootr==NULL)
        {
            if(rootl!=NULL||rootr!=NULL)
            {
                return false;
            }
            else return true;
        }
        
        if(rootl->val!=rootr->val)  return false;
        else
        {
            if(isSame(rootl->left,rootr->right))
            {
                if(isSame(rootl->right,rootr->left))
                {
                    return true;
                }
                else return false;
            }
            else return false;
        }
        
    }
};