0144.二叉树的前序遍历

/**
 * 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) {
  const ret = [];

  const preorder = (node) => {
    if (!node) return null;
    ret.push(node.val);
    preorder(node.left);
    preorder(node.right);
  };

  preorder(root);

  return ret;
};

/**
 * 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) {
  const ret = [];
  const stack = [];
  let node = root;
  while (node || stack.length > 0) {
    while (node) {
      ret.push(node.val);
      stack.push(node);
      node = node.left;
    }
    node = stack.pop();
    node = node.right;
  }
  return ret;
};