Problem

Analysis Process
Two kinds of solutions
One is Rescursive it is very easy
The other is Iterative
Start at the root node, each iteration pops the current top element and pushes its child onto the stack, pressing the right child first and then the left child

Code

Rescursive

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
var res []int

func preorderTraversal(root *TreeNode) []int {
	res = []int{}
	dfs(root)
	return res
}

func dfs(root *TreeNode) {
	if root != nil {
		res = append(res, root.Val)
		dfs(root.Left)
		dfs(root.Right)
	}
}

Iterative

func preorderTraversal(root *TreeNode) []int {
	var res []int
	var stack []*TreeNode

	for 0 < len(stack) || root != nil { //root != nil in order to determine the fist root must be put in the end
		for root != nil {
			res = append(res, root.Val)       //preorder traversal to pop
			stack = append(stack, root.Right) //ringt node push 
			root = root.Left                  //move to the left
		}
		index := len(stack) - 1 //top
		root = stack[index]     //pop
		stack = stack[:index]
	}
	return res
}