LeetCode 144. Binary Tree Preorder Traversal

输入：[1,null,2,3]

   1
    \
     2
    /
   3

输出：[1,2,3]

var preorderTraversal = function (root) {
  let result = [];
  preoderTraversalRecursive(root, result);

  return result;
};

var preoderTraversalRecursive = function (root, result) {
  if (root) {
	// 根节点
    result.push(root.val);

	// 左节点
    preoderTraversalRecursive(root.left, result);
	
	// 右节点
    preoderTraversalRecursive(root.right, result);
  }
};

var preoderTraversalIterative = function (root) {
  let result = [];

  if (root) {
    let list = [];
    list.push(root);
    while (list.length > 0) {
      // 取第一个数据
      const node = list.shift();
      result.push(node.val);

	  // 右节点进栈
      if (node.right) {
        list.unshift(node.right);
      }

	  // 左节点进栈
      if (node.left) {
        list.unshift(node.left);
      }
    }
  }

  return result;
};