leetcode 142

Author: Pankajcoder1

LeetCode/142.Linked List Cycle II.cpp

@@ -0,0 +1,56 @@
+/*
+    written by Pankaj Kumar.
+    country:-INDIA
+*/
+typedef long long ll ;
+const ll INF=1e18;
+const ll mod1=1e9+7;
+const ll mod2=998244353;
+//Add main code here
+
+/**
+ * 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)
+    {
+        // method 1
+        ListNode *slow = head, *fast = head;
+
+        while (fast != NULL && fast->next != NULL)
+        {
+            slow = slow->next; 
+            fast = fast->next->next;
+            if(slow == fast){
+                slow = head;
+                while(slow != fast){
+                    slow = slow->next;
+                    fast = fast->next;
+                }
+                return slow;
+            }
+        }
+        return NULL;
+
+        // method 2
+        ListNode *temp = head;
+        unordered_map<ListNode *, int> m;
+        while (temp != NULL)
+        {
+            if (m.find(temp) != m.end())
+            {
+                return temp;
+            }
+            m[temp]++;
+            temp = temp->next;
+        }
+        return NULL;
+    }
+};