my_solution.js

class TreeNode {
  constructor(val, left, right) {
    this.val = (val === undefined ? 0 : val)
    this.left = (left === undefined ? null : left)
    this.right = (right === undefined ? null : right)
  }
}
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
function invertTree(root) {
  if (null === root) return null;

  let left = root.left, right = root.right;

  if (null !== root.left) {
    root.right = left;
    // root.left = right;
    invertTree(root.right);
  }

  if (null !== root.right) {
    root.left = right;
    root.right = left;
    invertTree(root.left);
  }

  return root;
}
// function invertTree(root) {
//   if (null === root) return null;
//   if (null === root.val) return new TreeNode();
//
//   const invertNode = (node) => {
//     if (null === node) return node;
//     let left = node.left, right = node.right;
//
//     if (null !== node.left) {
//       node.right = left;
//       // node.left = right;
//       invertNode(node.right);
//     }
//
//     if (null !== node.right) {
//       node.left = right;
//       node.right = left;
//       invertNode(node.left);
//     }
//
//
//     return node;
//   }
//
//   return invertNode(root);
// }

//[4,2,7,1,3,6,9]
let x =
  // invertTree(new TreeNode(4,
  //   new TreeNode(2, new TreeNode(1), new TreeNode(3)),
  //   new TreeNode(7, new TreeNode(6), new TreeNode(9))
  // ));
  // invertTree(new TreeNode(1,
  //   new TreeNode(2)
  // ));
  invertTree(new TreeNode(1,
    null,
    new TreeNode(2)
  ));

console.log("Result")
console.log(x)