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

Note:
Bonus points if you could solve it both recursively and iteratively.

原本我的做法，是先求出一棵树的镜像，然后再查看这样的两棵树是不是相等的（isSameTree()请查看本博客leetcode-same tree）

但是卡在了{1,2}这个测试例子上，怎么都通不过。。我的代码如下：（失败）

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode *root1 = root;
        mirror(root1);
        return isSameTree(root,root1);
        
    }
    
    void mirror(TreeNode *root){
        if(root == NULL){
            return;
        }
        if(root->left == NULL && root->right == NULL){
            return;
        }
        //exchange
        TreeNode *temp = root->left;
        root->left = root->right;
        root->right = temp;
        
        if(root->left){
            mirror(root->left);
        }
        if(root->right){
            mirror(root->right);
        }
    }
    
    bool isSameTree(TreeNode *p, TreeNode *q) {
        if(p==NULL && q==NULL){
            return true;
        }
        else if(p==NULL || q==NULL){
            return false;
        }
        
        if(p->val == q->val){
            return isSameTree(p->left,q->left) && isSameTree(p->right,q->right);
        }
        else{
            return false;
        }
    }
};

后来查看解答，发现就是node->left->val == node->right->val;node->right->val == node->left->val的一个递归：所以就有如下代码，省去了求镜像的过程：

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return isSame(root,root);
        
    }
    
    bool isSame(TreeNode *p, TreeNode *q) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(p==NULL && q==NULL){
            return true;
        }
        else if(p==NULL || q==NULL){
            return false;
        }
        
        if(p->val == q->val){
            return isSame(p->left,q->right) && isSame(p->right,q->left);
        }
        else{
            return false;
        }
    }
};