原理：利用二叉树中空闲指针
1.当前节点cur无左子树，cur向右移动
2.cur如果有左子树，找左子树上最右的节点mostright
2.1如果mostright的右孩子为空，让其指向cur,然后cur向左移动
2.2如果mostright的右孩子为cur,mostright.right == null,cur向右移动。

总结：从当前节点出发，遍历其左子树，遍历完左子树后，回到当前节点，然后在遍历其右子树。

package basic_class_06;

public class Code_01_MorrisTraversal {


    //这个就相当于二叉树的前序遍历，中序遍历，后序遍历
    //public static void process(Node head){
        //if(head == null){
        //  return ;
        //}

        //System.out.println(head.value);

        //process(head.left);

        //System.out.println(head.value);

        //process(head.right);

        //Sysytem.out.println(head.value);
    //}

    public static class Node {
        public int value;
        Node left;
        Node right;

        public Node(int data) {
            this.value = data;
        }
    }

    public static void morrisIn(Node head) {
        if (head == null) {
            return;
        }
        Node cur1 = head;
        Node cur2 = null;
        while (cur1 != null) {
            cur2 = cur1.left;
            if (cur2 != null) {
                while (cur2.right != null && cur2.right != cur1) {
                    cur2 = cur2.right;
                }
                if (cur2.right == null) {
                    cur2.right = cur1;
                    cur1 = cur1.left;
                    continue;
                } else {
                    cur2.right = null;
                }
            }
            System.out.print(cur1.value + " ");
            cur1 = cur1.right;
        }
        System.out.println();
    }

    public static void morrisPre(Node head) {
        if (head == null) {
            return;
        }
        Node cur1 = head;
        Node cur2 = null;
        while (cur1 != null) {
            cur2 = cur1.left;
            if (cur2 != null) {
                while (cur2.right != null && cur2.right != cur1) {
                    cur2 = cur2.right;
                }
                if (cur2.right == null) {
                    cur2.right = cur1;
                    System.out.print(cur1.value + " ");
                    cur1 = cur1.left;
                    continue;
                } else {
                    cur2.right = null;
                }
            } else {
                System.out.print(cur1.value + " ");
            }
            cur1 = cur1.right;
        }
        System.out.println();
    }

    public static void morrisPos(Node head) {
        if (head == null) {
            return;
        }
        Node cur1 = head;
        Node cur2 = null;
        while (cur1 != null) {
            cur2 = cur1.left;
            if (cur2 != null) {
                while (cur2.right != null && cur2.right != cur1) {
                    cur2 = cur2.right;
                }
                if (cur2.right == null) {
                    cur2.right = cur1;
                    cur1 = cur1.left;
                    continue;
                } else {
                    cur2.right = null;
                    printEdge(cur1.left);
                }
            }
            cur1 = cur1.right;
        }
        printEdge(head);
        System.out.println();
    }

    public static void printEdge(Node head) {
        Node tail = reverseEdge(head);
        Node cur = tail;
        while (cur != null) {
            System.out.print(cur.value + " ");
            cur = cur.right;
        }
        reverseEdge(tail);
    }

    public static Node reverseEdge(Node from) {
        Node pre = null;
        Node next = null;
        while (from != null) {
            next = from.right;
            from.right = pre;
            pre = from;
            from = next;
        }
        return pre;
    }

    // for test -- print tree
    public static void printTree(Node head) {
        System.out.println("Binary Tree:");
        printInOrder(head, 0, "H", 17);
        System.out.println();
    }

    public static void printInOrder(Node head, int height, String to, int len) {
        if (head == null) {
            return;
        }
        printInOrder(head.right, height + 1, "v", len);
        String val = to + head.value + to;
        int lenM = val.length();
        int lenL = (len - lenM) / 2;
        int lenR = len - lenM - lenL;
        val = getSpace(lenL) + val + getSpace(lenR);
        System.out.println(getSpace(height * len) + val);
        printInOrder(head.left, height + 1, "^", len);
    }

    public static String getSpace(int num) {
        String space = " ";
        StringBuffer buf = new StringBuffer("");
        for (int i = 0; i < num; i++) {
            buf.append(space);
        }
        return buf.toString();
    }

    public static void main(String[] args) {
        Node head = new Node(4);
        head.left = new Node(2);
        head.right = new Node(6);
        head.left.left = new Node(1);
        head.left.right = new Node(3);
        head.right.left = new Node(5);
        head.right.right = new Node(7);
        printTree(head);
        morrisIn(head);
        morrisPre(head);
        morrisPos(head);
        printTree(head);

    }

}