Morris遍历是一种时间复杂度O(n)的二叉树遍历。
Morris遍历与递归版本的二叉树遍历的关系(或者相似之处)，如果一个节点有左子树那么Morris遍历可以回到左子树两次(递归方法是3次)，Morris模拟了递归的过程。对于没有左子树的节点只会到达依一次。
Morris遍历(先序)的代码：
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| 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();
}
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Morris遍历(中序)的代码：
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| public static void morrisIn(Node head) {
if (head == null) {
return;
}
Node cur = head;
Node mostRight = null;
while (cur != null) {
mostRight = cur.left;
if (mostRight != null) {
while (mostRight.right != null && mostRight.right != cur) {
mostRight = mostRight.right;
}
if (mostRight.right == null) {
mostRight.right = cur;
cur = cur.left;
continue;
} else {
mostRight.right = null;
}
}
System.out.print(cur.value + " ");
cur = cur.right;
}
System.out.println();
}
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注：后序遍历有点麻烦，因为Morris遍历最多只能回到一个节点两次并不会回到第三次。采用下面的方法打印后序遍历：所有会回到两次的节点，在它第二次回到的时候逆序打印它的左子树右边界，打印完后，单独打印整棵树的右边界，这就后序遍历。
只关注能来到两次的节点。
Morris遍历(后序)的代码：
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| 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);
}
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