S590

leetcode/README.md

@@ -306,6 +306,7 @@
 | 572 | [Subtree of Another Tree](tree/isSubtree.go) | [Easy][] |
 | 575 | [Distribute Candies](hash-table/distributeCandies.go) | [Easy][] |
 | 581 | [Shortest Unsorted Continuous Subarray](array/findUnsortedSubarray.go) | [Easy][] |
+| 590 | [N-ary Tree Postorder Traversal](tree/postorder.go) | [Easy][] |
 | 594 | [Longest Harmonious Subsequence](hash-table/findLHS.go) | [Easy][] |
 | 598 | [Range Addition II](math/maxCount.go) | [Easy][] |
 | 599 | [Minimum Index Sum of Two Lists](hash-table/findRestaurant.go) | [Easy][] |

leetcode/tree/postorder.go

@@ -0,0 +1,61 @@
+/* https://leetcode.com/problems/n-ary-tree-postorder-traversal/
+Given the root of an n-ary tree, return the postorder traversal of its nodes' values.
+
+Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)
+
+Example 1:
+
+Input: root = [1,null,3,2,4,null,5,6]
+Output: [5,6,3,2,4,1]
+Example 2:
+
+Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
+Output: [2,6,14,11,7,3,12,8,4,13,9,10,5,1]
+
+Constraints:
+
+The number of nodes in the tree is in the range [0, 10^4].
+0 <= Node.val <= 10^4
+The height of the n-ary tree is less than or equal to 1000.
+*/
+
+package ltree
+
+type Node struct {
+	Val      int
+	Children []*Node
+}
+
+func postorder(root *Node) []int {
+	// var helper func(root *Node, ans *[]int)
+	// helper = func(root *Node, ans *[]int) {
+	// 	if root == nil {
+	// 		return
+	// 	}
+	// 	for _, child := range root.Children {
+	// 		helper(child, ans)
+	// 	}
+	// 	*ans = append(*ans, root.Val)
+	// }
+
+	// ans := []int{}
+	// helper(root, &ans)
+	// return ans
+
+	ans := []int{}
+	stack := []*Node{root}
+	for len(stack) != 0 {
+		cur := stack[len(stack)-1]
+		ans = append(ans, cur.Val)
+		stack = stack[:len(stack)-1]
+		for _, child := range cur.Children {
+			stack = append(stack, child)
+		}
+	}
+
+	for i, j := 0, len(ans)-1; i < j; i, j = i+1, j-1 {
+		ans[i], ans[j] = ans[j], ans[i]
+	}
+
+	return ans
+}

leetcode/tree/postorder_test.go

@@ -0,0 +1,34 @@
+package ltree
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_postorder(t *testing.T) {
+	assert := assert.New(t)
+
+	// [1,null,3,2,4,null,5,6]
+	root :=
+		&Node{
+			Val: 1,
+			Children: []*Node{&Node{
+				Val: 3,
+				Children: []*Node{&Node{
+					Val:      5,
+					Children: []*Node{},
+				}, &Node{
+					Val:      6,
+					Children: []*Node{},
+				}},
+			}, &Node{
+				Val:      2,
+				Children: []*Node{},
+			}, &Node{
+				Val:      4,
+				Children: []*Node{},
+			}},
+		}
+	assert.Equal([]int{5, 6, 3, 2, 4, 1}, postorder(root))
+}