Symmetric Tree.cpp

/*
Symmetric Tree
==============

Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:

The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.
 

Example 1:
    2
   / \
  1   3

Input: [2,1,3]
Output: true

Example 2:
    5
   / \
  1   4
     / \
    3   6

Input: [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.
*/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

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

public:
  bool isSymmetric(TreeNode *root)
  {
    if (!root)
      return true;
    return isSymmetric(root->left, root->right);
  }
};