# 101. Symmetric Tree

## 1.問題 

* 給予一個binary tree, 判斷是否為對稱

![](/files/-LOSXHJR-RQAxeLZSNpp)

## 2.想法 <a href="#id-2-xiang-fa" id="id-2-xiang-fa"></a>

* 提問
  * 確認題意:  root是NULL是true或false?
* function header, parameter
* test input
* 說明想法
  * 將root複製出一份左右對稱的樹後, 比較root 與該數是否是same tree
* 測試計算複雜度
  * recursive的runtime是計算total call number
    * deepCopy
    * swap
    * isSameTree

## **3.程式碼** <a href="#id-3-cheng-shi" id="id-3-cheng-shi"></a>

```
/**
 * 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;
        }
        
        TreeNode* copy = deepCopy(root);
        TreeNode* swapNode = swap(copy);
        return isSameTree(root, swapNode);
    }
private:
    TreeNode* deepCopy(TreeNode* node){
        if (!node) {
            return NULL;
        }
        TreeNode* newNode = new TreeNode(node->val);
        newNode->left = deepCopy(node->left);
        newNode->right = deepCopy(node->right);
        return newNode;
    }
    
    TreeNode* swap(TreeNode* node){
        if (!node) {
            return NULL;
        }
        
        node->left = swap(node->left);
        node->right = swap(node->right);
        TreeNode* tmp = node->left;
        node->left = node->right;
        node->right = tmp;
        return node;
    }
    
    bool isSameTree(TreeNode* p, TreeNode* q) {
        if (!p && !q) {
            return true;
        } 
        
        if (!p || !q) {
            return false;
        } 
        
        if (p->val == q->val) {
            return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
        }
        
        return false;
    }
};
```


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