Basic Questions on LinkedList

TC: O(N)

/**
 * 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 Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null) return null; 
        ListNode current = head;
        ListNode pre = null;
        ListNode next = current.next;
        while(current!= null){
            current.next = pre;
            pre = current;
            current = next;
            if(next!= null) next = next.next;
        }
        return pre;
    }
}

Or it can also done as (without using next pointer)

/**
 * 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 Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode current  = head;
        while(head!=null){
            head = head.next;
            current.next  = prev;
            prev = current;
            current = head;
        }
        return prev;
    }
}

Delete a node in a linkedlist when the head of the node is not given in O(1)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public void deleteNode(ListNode node) {
        node.val = node.next.val;
        node.next = node.next.next;
    }
}

Delete nth node from the back of the linkedlist

TC:O(N)

/**
 * 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 Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        // we will use fast and slow pointers for solving this problem 
        //for avoiding edge case if there is only one node in the linkedList
        ListNode start = new ListNode();
        start.next = head; // by this way we are ensuring that length of the list is atleast 2
        ListNode fast = start;
        ListNode slow = start;
        for(int i =1;i<=n;++i){
            fast =fast.next;
        }
        while(fast.next!=null){
            fast = fast.next;
            slow = slow.next;
        }
        slow.next = slow.next.next;
        return start.next;
    }
}

Merge to sorted list
TC: O(n) ,where n is min(len(list1),len(list2))

/**
 * 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 Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode temp = new ListNode(0);
        ListNode current = temp;
        while(l1!=null && l2 != null){
            if(l1.val < l2.val){
                current.next = l1;
                current = l1;
                l1 = l1.next;
            }
            else {
                current.next = l2;
                current  = l2;
                l2  = l2.next;
            }
        }
        if(l1==null) current.next = l2;
        else current.next = l1;
        return temp.next;
    }
}