给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

示例 1：

输入：root = [1,null,2,3]
输出：[1,2,3]

/**
 * 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) {
    let ans = []

    function firstTravser(node) {
        if(node !== null) {
            ans.push(node.val)
            firstTravser(node.left)
            firstTravser(node.right)
        }
    }

    firstTravser(root)

    return ans
};

/**
 * 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) {
    let ans = [];
	let stack = [];

	while (stack.length > 0 || root !== null) {
		while (root !== null) {
			ans.push(root.val);
			stack.push(root);
			root = root.left;
		}

		let next = stack.pop();
		root = next.right;
	}

	return ans;
};

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/binary-tree-preorder-traversal/