fd

Author: dingjikerbo

solution/src/main/java/com/inuker/solution/LinkedListCycle.java

@@ -10,18 +10,13 @@
 public class LinkedListCycle {
 
     public boolean hasCycle(ListNode head) {
-        if (head == null) {
-            return false;
-        }
-
-        ListNode fast = head.next, slow = head;
-
-        for ( ; fast != null && fast.next != null; fast = fast.next.next, slow = slow.next) {
+        for (ListNode fast = head, slow = head; fast != null && fast.next != null; ) {
+            slow = slow.next;
+            fast = fast.next.next;
             if (fast == slow) {
                 return true;
             }
         }
-
         return false;
     }
 }

solution/src/main/java/com/inuker/solution/LinkedListCycleII.java

@@ -7,31 +7,26 @@
  */
 
 public class LinkedListCycleII {
-
+    /**
+     * 这里有个问题，如果链表是a,b，然后又回到a
+     * 则slow和fast必须都从head起步才行
+     * 如果slow从head起步，fast从head.next起步，则这里会死循环
+     */
     public ListNode detectCycle(ListNode head) {
-        if (head == null || head.next == null) return null;
-
-        ListNode firstp = head;
-        ListNode secondp = head;
-        boolean isCycle = false;
-
-        while (firstp != null && secondp != null) {
-            firstp = firstp.next;
-            if (secondp.next == null) return null;
-            secondp = secondp.next.next;
-            if (firstp == secondp) {
-                isCycle = true;
+        ListNode slow = head, fast = head;
+        boolean hasCycle = false;
+        for (; fast != null && fast.next != null; ) {
+            slow = slow.next;
+            fast = fast.next.next;
+            if (slow == fast) {
+                hasCycle = true;
                 break;
             }
         }
-
-        if (!isCycle) return null;
-        firstp = head;
-        while (firstp != secondp) {
-            firstp = firstp.next;
-            secondp = secondp.next;
+        if (!hasCycle) {
+            return null;
         }
-
-        return firstp;
+        for (ListNode node = head; node != slow; node = node.next, slow = slow.next);
+        return slow;
     }
 }

test/src/main/java/com/inuker/test/main.java

@@ -24,21 +24,14 @@ public class main {
     public static void main(String[] args) {
     }
 
-
-    public ListNode insertionSortList(ListNode head) {
-        ListNode dummy = new ListNode(0);
-        for (ListNode next; head != null; head = next) {
-            next = head.next;
-            for (ListNode cur = dummy; cur != null; cur = cur.next) {
-                if (cur.next != null && head.val > cur.next.val) {
-                    continue;
-                }
-                head.next = cur.next;
-                cur.next = head;
-                break;
+    public boolean hasCycle(ListNode head) {
+        for (ListNode fast = head, slow = head; fast != null && fast.next != null; ) {
+            slow = slow.next;
+            fast = fast.next.next;
+            if (fast == slow) {
+                return true;
             }
         }
-        return dummy.next;
+        return false;
     }
-
 }