[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

Recursive: Pre-order comparison

class Solution {
public:
    bool isSymmetric(TreeNode* left, TreeNode* right) {
        if(!left || !right) return left == right;
        return left->val == right->val && 
               isSymmetric(left->left, right->right) && 
               isSymmetric(left->right, right->left);
    }
    bool isSymmetric(TreeNode* root) {
        return isSymmetric(root, root);
    }
};