LeetCode 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 is symmetric:

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

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

    static bool isSymmetric(TreeNode* left, TreeNode* right) {
        if(!left && !right) return true;
        if(!left && right) return false;
        if(!right && left) return false;
        return left->val == right->val && isSymmetric(left->left, right->right) && isSymmetric(left->right, right->left);
    }
    bool isSymmetric(TreeNode* root) {
        if(!root) return true;
        return isSymmetric(root->left, root->right);
    }