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// @Title: 二叉树的前序遍历 (Binary Tree Preorder Traversal)
// @Author: 15816537946@163.com
// @Date: 2020-06-25 07:15:36
// @Runtime: 0 ms
// @Memory: 2.1 MB
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
 /*
func preorderTraversal(root *TreeNode) []int {
	if root == nil {
		return  nil
	}
	res := make([]int,0)
	var dfs func(*TreeNode)
	dfs = func(node *TreeNode) {
		if node == nil {
			return 
		}
		res = append(res, node.Val)
		dfs(node.Left)
		dfs(node.Right)
	}
	dfs(root)

	return res
}
*/
func preorderTraversal(root *TreeNode) []int {
	if root == nil {
		return nil
	}
	res := make([]int, 0)
	nodes := make([]*TreeNode, 1, 1024)
	nodes[0] = root
	for len(nodes) > 0 {
		node := nodes[len(nodes)-1]
		nodes = nodes[:len(nodes)-1]
		res = append(res, node.Val)
		if node.Right != nil {
			nodes = append(nodes, node.Right)
		}
		if node.Left != nil {
			nodes = append(nodes, node.Left)
		}
	}

	return res
}