Merge pull request neetcode-gh#2057 from andrewmustea/add_0142-linked…

…-list-cycle-ii.c

create 0142-linked-list-cycle-ii.c

c/0142-linked-list-cycle-ii.c

@@ -0,0 +1,59 @@
+/**
+ * Given the head of a linked list, return the node where the cycle begins. If
+ * there is no cycle, return null.
+ *
+ * There is a cycle in a linked list if there is some node in the list that can
+ * be reached again by continuously following the next pointer. Internally, pos
+ * is used to denote the index of the node that tail's next pointer is connected
+ * to (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as
+ * a parameter.
+ *
+ * Do not modify the linked list.
+ *
+ * Constraints:
+ *
+ * The number of the nodes in the list is in the range [0, 104].
+ * -105 <= Node.val <= 105
+ * pos is -1 or a valid index in the linked-list.
+ *
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ *
+ * Space: O(1)
+ * Time: O(n)
+ */
+
+struct ListNode *detectCycle(struct ListNode *head) {
+    struct ListNode *fast = head;
+    struct ListNode *slow = head;
+
+    if (!head)
+        return NULL;
+
+    while (fast->next && fast->next->next) {
+        fast = fast->next->next;
+        slow = slow->next;
+
+        if (fast == slow) {
+            /**
+             * The index of the node cycle is located the same number of nodes
+             * away from the start of the linked list and the intersection of
+             * the slow and fast pointers.
+             */
+            struct ListNode *head_node = head;
+            struct ListNode *intersection = slow;
+
+            while (head_node != intersection) {
+                head_node = head_node->next;
+                intersection = intersection->next;
+            }
+
+            return intersection;
+        }
+    }
+
+    return NULL;
+}