144. 二叉树的前序遍历

题目

给定一个二叉树，返回它的前序遍历。

示例:

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,2,3]

题解

递归

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
  let ans = [];
  const preOrder = root => {
    if (!root) return;
    ans.push(root.val);
    preOrder(root.left);
    preOrder(root.right);
  };
  preOrder(root);
  return ans;
};

迭代

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
  let ans = [];
  if (!root) return ans;
  let s = [root];
  while (s.length) {
    let node = s.pop();
    ans.push(node.val);
    if (node.right) s.push(node.right);
    if (node.left) s.push(node.left);
  }
  return ans;
};