簡單的遞歸，不過這次是用ts寫的，希望越寫越會！

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    this.val = val === undefined ? 0 : val;
    this.left = left === undefined ? null : left;
    this.right = right === undefined ? null : right;
  }
}
function preorderTraversal(root: TreeNode | null): number[] {
  let result: number[] = [];
  function dfs(root: TreeNode | null): void {
    if (!root) {
      return;
    }
    result.push(root.val);
    if (root.left) {
      dfs(root.left);
    }
    if (root.right) {
      dfs(root.right);
    }
  }
  dfs(root);
  return result;
}