101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3 

簡單來說，判斷是否為對稱樹

程式碼 

class Solution {
public:
    bool isSymmetric(TreeNode* root) 
    {
        
        if (root == NULL) return true;
        
        return isSymc(root->left, root->right);
        
    }
    
    bool isSymc(TreeNode* left, TreeNode* right)
    {
        if (left == nullptr)
        {
            return right == nullptr;
        }
        
        if (right == nullptr)
        {
            return left == nullptr;
        }
        
        if (left->val != right->val)
        {
            return false;
        }
        
        // 檢查是否對稱
        if (!isSymc(left->right, right->left))
        {
            return false;
        }
        
        if (!isSymc(left->left, right->right))
        {
            return false;
        }
            
        return true;
    }
    
};