[Day27]-[tree] 遍历

func TestPreorderTraversal(t *testing.T) {	tree := &TreeNode{		Val: 2,		Left: &TreeNode{			Val: 1,		},		Right: &TreeNode{			Val: 3,		},	}	var preorderTraversal func(root *TreeNode) []int	var res []int	preorderTraversal = func(root *TreeNode) []int {		if root == nil {			return res		}
		res = append(res, root.Val)		preorderTraversal(root.Left)		preorderTraversal(root.Right)		return res	}
	t.Log(preorderTraversal(tree))}

func inorderTraversal(root *TreeNode) []int {	var res []int    var dfs func(root *TreeNode) []int	dfs = func(root *TreeNode) []int {		if root == nil {			return res		}		dfs(root.Left)        res = append(res, root.Val)		dfs(root.Right)		return res	}
    return dfs(root)}

func postorderTraversal(root *TreeNode) []int {	var res []int    var dfs func(root *TreeNode) []int	dfs = func(root *TreeNode) []int {		if root == nil {			return res		}
		dfs(root.Left)		dfs(root.Right)        res = append(res, root.Val)		return res	}
    return dfs(root)}