isSymetricTree

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

// YES -- [1,2,2,3,4,4,3]
//      1
//    /   \
//   2     2
//  / \   / \
// 3   4 4   3  

// NO -- [1,2,2,null,3,null,3]
//    1
//  /   \
// 2     2
//  \     \
//   3     3

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
// How to solve this problem:
// 1. Check if the tree is empty or has only a root node.
// 2. If not, reverse the right subtree.
// 3. Check if the left and right subtrees are identical.
export default function isSymmetricTree(root) {
  if (root === null || (root.right === null && root.left === null)) {
    return true;
  }

  // reverse tree on the right
  root.right = revertTree(root.right);
  return isSameTree(root.left, root.right);
}

// Function to reverse tree
function revertTree(node) {
  if (node === null || (node.left === null && node.right === null)) {
    return node;
  }
  const temp = revertTree(node.left); // temp -> left (reverse child)
  node.left = revertTree(node.right); // left -> right (reverse child)
  node.right = temp; // right -> temp
  return node;
}

// function checking similar tree
function isSameTree(left, right) {
  if (left === null && right === null) {
    return true;
  }
  if (left === null || right === null) {
    return false;
  }
  if (left.val !== right.val) {
    return false;
  }
  return isSameTree(left.right, right.right) && isSameTree(left.left, right.left);
}