[LeetCode從零單刷]Symmetric Tree

[LeetCode從零單刷]Symmetric Tree

題目：

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

最開始非常腦殘地希望補全二叉樹，然後利用1，2，4，8個節點之間的關系來比較。結果越來越復雜。

還是那句話：碰上樹，能遞歸就遞歸，不能遞歸就棧、隊列。這次的遞歸要點：某節點的左子樹的左子樹 = 右子樹的右子樹，左子樹的右子樹 = 右子樹的左子樹。

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