/**
 * Definition for a binary tree node.
 * function TreeNode(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 {number[]}
 */
var preorderTraversal = function(root) {
  if(root == null){
    return [];
  }
  let res = [];
  let cur = root;
  while(cur){
    if(cur.left == null){
      res.push(cur.val);
      cur = cur.right;
    }else{
      let node = cur.left;
      while(node != null && node.right != null && node.right != cur){
        node = node.right;
      }
      if(node.right == null){
        res.push(cur.val);
        node.right = cur;
        cur = cur.left;
      }else{
        node.right = null;
        cur = cur.right;
      }
    }
  }
  return res;
};