AC 145

hard/145.go

@@ -8,10 +8,52 @@ package main
  *     Right *TreeNode
  * }
  */
-func postorderTraversal(root *TreeNode) []int {
-	return []int{}
+func postorderTraversal(root *TreeNode) (nodes []int) {
+	if root == nil {
+		return
+	}
+	return postorderIteration(root)
 }
 
-func main() {
+func postorderRecurse(root *TreeNode, nodes *[]int) {
+	if root == nil {
+		return
+	}
+	postorderRecurse(root.Left, nodes)
+	postorderRecurse(root.Right, nodes)
+	*nodes = append(*nodes, root.Val)
+}
 
+func postorderIteration(root *TreeNode) (nodes []int) {
+	stack := make([]*TreeNode, 0)
+	for curr := root; ; {
+		for curr != nil {
+			if curr.Right != nil {
+				stack = append(stack, curr.Right)
+			}
+			stack = append(stack, curr)
+			curr = curr.Left
+		} // Save right, root; visit left (the right node is saved to mark root.Right as unvisited)
+		// curr is nil, pop top of stack
+		curr = stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		// If right is not visited, pop right, save root again, and visit right
+		if curr.Right != nil && len(stack) != 0 && curr.Right == stack[len(stack)-1] {
+			stack = stack[:len(stack)-1]
+			stack = append(stack, curr)
+			curr = curr.Right
+		} else { // Else print root
+			nodes = append(nodes, curr.Val)
+			curr = nil
+		}
+		if len(stack) == 0 {
+			break
+		}
+	}
+	return
 }
+
+// func main() {
+// 	fmt.Println(postorderTraversal(
+// 		toBinaryTree(newInt(1), nil, newInt(2), newInt(3))))
+// }