Solution.go

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func preorderTraversal(root *TreeNode) []int {
	var res []int
	for root != nil {
		if root.Left == nil {
			res = append(res, root.Val)
			root = root.Right
		} else {
			pre := root.Left
			for pre.Right != nil && pre.Right != root {
				pre = pre.Right
			}
			if pre.Right == nil {
				res = append(res, root.Val)
				pre.Right = root
				root = root.Left
			} else {
				pre.Right = nil
				root = root.Right
			}
		}
	}
	return res
}