Solution3.go

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