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Author: assassint2017

Linked List Cycle II/Linked_List_Cycle_II.cpp

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+// 双指针
+// Runtime: 12 ms, faster than 29.54% of C++ online submissions for Linked List Cycle II.
+// Memory Usage: 6.7 MB, less than 0.89% of C++ online submissions for Linked List Cycle II.
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution 
+{
+public:
+    ListNode *detectCycle(ListNode *head) 
+    {
+        if (head == 0)
+            return 0;
+
+        ListNode *fastPtr = head, *slowPtr = head;
+        
+        bool hasCycle = false;
+        while (slowPtr->next != 0 && fastPtr->next != 0 && fastPtr->next->next !=0)
+        {
+            // 慢指针每次一共一格，快指针每次移动两格
+            slowPtr = slowPtr->next;
+            fastPtr = fastPtr->next->next;
+            if (fastPtr == slowPtr)
+            {
+                hasCycle = true;
+                break;
+            }
+        }
+
+        if (hasCycle == false)
+            return 0;
+
+        // 此时链表中存在环
+        fastPtr = head;
+        while(slowPtr != fastPtr)
+        {
+            slowPtr = slowPtr->next;
+            fastPtr = fastPtr->next;
+        }
+        return fastPtr;
+    }
+};

Linked List Cycle II/Linked_List_Cycle_II.py

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+# Runtime: 44 ms, faster than 99.63% of Python online submissions for Linked List Cycle II.
+# Memory Usage: 16.2 MB, less than 94.55% of Python online submissions for Linked List Cycle II.
+
+# Definition for singly-linked list.
+# class ListNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution(object):
+    def detectCycle(self, head):
+        """
+        :type head: ListNode
+        :rtype: ListNode
+        """
+        if head is None:
+            return None
+        fastPtr, slowPtr = head, head
+        hasCycle = False
+        while (slowPtr.next is not None and fastPtr.next is not None and fastPtr.next.next is not None):
+            slowPtr = slowPtr.next
+            fastPtr = fastPtr.next.next
+            if slowPtr == fastPtr:
+                hasCycle = True
+                break
+        if not hasCycle:
+            return None
+        fastPtr = head
+        while fastPtr != slowPtr:
+            slowPtr = slowPtr.next
+            fastPtr = fastPtr.next
+        return fastPtr