0、二叉樹結點的構造

1 public static class Node {
2         public int value;
3         public Node left;
4         public Node right;
5 
6         public Node(int data) {
7             this.value = data;
8         }
9 }

1、前序的非遞迴遍歷：

棧中每次彈出的當前結點後：列印彈出結點val值，有右先壓右，有左後壓左

 1 public static void preOrderUnRecur(Node head) {
 
 2     System.out.print("pre-order: ");
 3     if (head != null) {
 4         Stack<Node> stack = new Stack<Node>();
 5         stack.add(head);
 6         while (!stack.isEmpty()) {
 7             head = stack.pop();
 8             System.out.print(head.value + " ");
 9             if (head.right != null
 
) {
10                 stack.push(head.right);
11             }
12             if (head.left != null) {
13                 stack.push(head.left);
14             }
15         }
16     }
17     System.out.println();
18 }

2、中序的非遞迴遍歷

當前結點不為空：壓棧，當前結點往左走

當前結點為空：彈棧，列印彈出棧的val值，當前結點往右走

 1 public static void
 
 inOrderUnRecur(Node head) {
 2     System.out.print("in-order: ");
 3     if (head != null) {
 4         Stack<Node> stack = new Stack<Node>();
 5         while (!stack.isEmpty() || head != null) {
 6             if (head != null) {
 7                 stack.push(head);
 8                 head = head.left;
 9             } else {
10                 head = stack.pop();
11                 System.out.print(head.value + " ");
12                 head = head.right;
13             }
14         }
15     }
16     System.out.println();
17 }

3、後序的非遞迴遍歷

已知前序是：中--左--右，先轉換成：中--右--左，然後申請一個輔助棧顛倒成：左--右--中，接著遍歷列印輔助棧內結點的val值

那麼如何使用輔助棧顛倒成：左--右--中呢？

在彈棧的時候不列印元素而是將其壓入輔助棧

 1 public static void posOrderUnRecur(Node head) {
 2     System.out.print("pos-order: ");
 3     if (head != null) {
 4         Stack<Node> s1 = new Stack<Node>();
 5         Stack<Node> s2 = new Stack<Node>();
 6         s1.push(head);
 7         while (!s1.isEmpty()) {
 8             head = s1.pop();
 9             s2.push(head);
10             if (head.left != null) {
11                 s1.push(head.left);
12             }
13             if (head.right != null) {
14                 s1.push(head.right);
15             }
16         }
17         while (!s2.isEmpty()) {
18             System.out.print(s2.pop().value + " ");
19         }
20     }
21     System.out.println();
22 }