feat(590): Easy

Same as https://github.com/DziedzicGrzegorz/GoPatterns/blob/55ac301d7124fa145b79d5fb8ed754cca02fbde4/internal/Leetcode/145/145.go
but multiple nodes

BREAKING CHANGE: Remember the following text: "It can be faster with a non-recursive solution.

internal/Leetcode/590/590.go

@@ -0,0 +1,21 @@
+package _590
+
+type Node struct {
+	Val      int
+	Children []*Node
+}
+
+func postorder(root *Node) (ans []int) {
+	var dfs func(*Node)
+	dfs = func(root *Node) {
+		if root == nil {
+			return
+		}
+		for _, child := range root.Children {
+			dfs(child)
+		}
+		ans = append(ans, root.Val)
+	}
+	dfs(root)
+	return
+}

internal/Leetcode/590/590_test.go

@@ -0,0 +1,50 @@
+package _590
+
+import (
+	"reflect"
+	"testing"
+)
+
+func Test_postorder(t *testing.T) {
+	type args struct {
+		root *Node
+	}
+	tests := []struct {
+		name string
+		args args
+		want []int
+	}{
+		{
+			name: "Example 1",
+			args: args{
+				root: &Node{
+					Val: 1,
+					Children: []*Node{
+						{
+							Val: 3,
+							Children: []*Node{
+								{
+									Val:      5,
+									Children: nil,
+								},
+								{
+
+									Val:      6,
+									Children: nil,
+								},
+							},
+						},
+					},
+				},
+			},
+			want: []int{5, 6, 3, 1},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := postorder(tt.args.root); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("postorder() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}