所謂Morris遍歷，就是建立線索二叉樹，然後在遍歷的同時邊建邊拆。

主要思路就是，我先拿到當前節點，然後設定一個mostRight指向當前節點的左子樹，然後不斷地找右子樹（這樣做是為了找到當前節點的直接前驅節點）。找到之後，如果這個節點的右孩子指向為空，則指向當前節點，這就是建立線索二叉樹；同時，為了不破壞二叉樹的結構，當這個節點右孩子指向為當前節點時，我們就要把線索斷掉，這就是拆。

左子樹遍歷完後，我們就需要遍歷右子樹。

具體可以看看視訊教程：

Morris遍歷

我們直接來看程式碼：

  1 package com.hw.list0710;
  2 
  3 public class Morris {
 
  4     static class TreeNode{
  5         TreeNode left;
  6         TreeNode right;
  7         int val;
  8         public TreeNode(int val, TreeNode left, TreeNode right){
  9             this.val = val;
 10             this.left = left;
 11             this.right = right;
 12         }
 13     }
 
 14 
 15     /**
 16      * 先序遍歷線索二叉樹
 17      * @param cur
 18      */
 19     private static void morrisPre(TreeNode cur){
 20         if(cur == null){
 21             return;
 22         }
 23         TreeNode mostRight = null;
 24         while(cur != null)
 25         {
 26             mostRight = cur.left;
 
 27             if(mostRight != null){
 28                 while(mostRight.right != null && mostRight.right != cur)
 29                 {
 30                     mostRight = mostRight.right;
 31                 }
 32                 if(mostRight.right == null){  //建立線索指標
 33                     mostRight.right = cur;
 34                     System.out.print(cur.val+" ");
 35                     cur = cur.left;
 36                     continue;
 37                 }else{
 38                     mostRight.right = null;  //刪除線索指標
 39                 }
 40             }else{
 41                 System.out.print(cur.val+" ");
 42             }
 43             cur = cur.right;  //左邊沒了，要到右邊；左邊遍歷完了，也要到右邊
 44         }
 45     }
 46 
 47     /**
 48      * 中序遍歷線索二叉樹
 49      * @param cur
 50      */
 51     private static void morrisMid(TreeNode cur){
 52         if(cur == null){
 53             return;
 54         }
 55         TreeNode mostRight = null;
 56         while(cur != null)
 57         {
 58             mostRight = cur.left;
 59             if(mostRight != null){
 60                 while(mostRight.right != null && mostRight.right != cur)
 61                 {
 62                     mostRight = mostRight.right;
 63                 }
 64                 if(mostRight.right == null){
 65                     mostRight.right = cur;
 66                     cur = cur.left;
 67                     continue;
 68                 }else{
 69                     System.out.print(cur.val+" ");
 70                     mostRight.right = null;
 71                 }
 72             }else{
 73                 System.out.print(cur.val+" ");
 74             }
 75             cur = cur.right;
 76         }
 77     }
 78 
 79     /**
 80      * 反轉連結串列
 81      * @param head
 82      * @return
 83      */
 84     private static TreeNode reverse(TreeNode head){
 85         TreeNode pre = null;
 86         TreeNode cur, next;
 87         cur = head;
 88         while(cur != null)
 89         {
 90             next = cur.right;
 91             cur.right = pre;
 92             pre = cur;  //指向當前節點的上一個節點的指標前移
 93             cur = next;
 94         }
 95         return pre;
 96     }
 97 
 98     /**
 99      * 列印連結串列節點
100      * @param head
101      */
102     private static void printNode(TreeNode head){
103         TreeNode tail = reverse(head);
104         while(tail != null)
105         {
106             System.out.print(tail.val+" ");
107             tail = tail.right;
108         }
109         reverse(tail);
110     }
111 
112     /**
113      * 後序遍歷線索二叉樹
114      * @param cur
115      */
116     private static void morrisLast(TreeNode cur){
117         if(cur == null){
118             return;
119         }
120         TreeNode root = cur;
121         TreeNode mostRight = null;
122         while(cur != null)
123         {
124             mostRight = cur.left;
125             if(mostRight != null){
126                 while(mostRight.right != null && mostRight.right != cur)
127                 {
128                     mostRight = mostRight.right;
129                 }
130                 if(mostRight.right == null){  
131                     mostRight.right = cur;
132                     cur = cur.left;
133                     continue;
134                 }else{
135                     mostRight.right = null;  
136                     printNode(cur.left);
137                 }
138             }
139             cur = cur.right;  
140         }
141         printNode(root);
142     }
143 
144     public static void main(String[] args) {
145         TreeNode node6 = new TreeNode(6, null, null);
146         TreeNode node7 = new TreeNode(7, null, null);
147         TreeNode node5 = new TreeNode(5, node6, node7);
148         TreeNode node4 = new TreeNode(4, null, null);
149         TreeNode node2 = new TreeNode(2, node4, node5);
150         TreeNode node3 = new TreeNode(3, null, null);
151         TreeNode node1 = new TreeNode(1, node2, node3);
152         System.out.print("先序遍歷:");
153         morrisPre(node1);
154         System.out.println();
155         System.out.print("中序遍歷:");
156         morrisMid(node1);
157         System.out.println();
158         System.out.print("後序遍歷:");
159         morrisLast(node1);
160     }
161 }