Linked List 1

Problem1.java

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+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Problem1 {
+    public ListNode reverseList(ListNode head) {
+        ListNode prev = null;
+        ListNode curr = head;
+
+        while(curr != null){
+            ListNode temp = curr.next;
+            curr.next = prev;
+            prev = curr;
+            curr = temp;
+        }
+
+        return prev;
+    }
+}

Problem2.java

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+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Problem2 {
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        ListNode dummy = new ListNode(-1);
+        dummy.next = head;
+
+        ListNode slow = dummy;
+        ListNode fast = dummy;
+
+        int count = 0;
+
+        while(count <= n){
+            fast = fast.next;
+            count++;
+        }
+
+        while(fast != null){
+            slow = slow.next;
+            fast = fast.next;
+        }
+
+        ListNode temp = slow.next;
+        slow.next = slow.next.next;
+        temp.next = null;
+
+        return dummy.next;
+    }
+}

Problem3.java

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+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) {
+ *         val = x;
+ *         next = null;
+ *     }
+ * }
+ */
+public class Problem3 {
+    public ListNode detectCycle(ListNode head) {
+        if(head == null) return null;
+        ListNode slow = head;
+        ListNode fast = head;
+
+        boolean flag = false;
+
+        while(fast.next != null && fast.next.next != null){
+            slow = slow.next;
+            fast = fast.next.next;
+
+            if(slow == fast){
+                flag = true;
+                break;
+            }
+        }
+
+        if(!flag) return null;
+
+        slow = head;
+        while(slow != fast){
+            slow = slow.next;
+            fast = fast.next;
+        }
+
+        return slow;
+    }
+}