Create 0061-rotate-list.md

dsa-solutions/lc-solutions/0000-0099/0061-rotate-list.md

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+---
+id: rotate-list
+title: Rotate List (LeetCode)
+sidebar_label: 0061-rotate-list
+tags:
+    - Linked List
+    - Two Pointers
+description: "The rotate list is the problem to rotate the list to the right by k places."
+---
+
+### Problem Description
+
+Given the `head` of a linked list, rotate the list to the right by `k` places.
+
+### Examples
+
+#### Example 1
+
+```plaintext
+Input: head = [1,2,3,4,5], k = 2
+Output: [4,5,1,2,3]
+```
+
+#### Example 2
+
+```plaintext
+Input: head = [0,1,2], k = 4
+Output: [2,0,1]
+```
+
+### Constraints
+
+- The number of nodes in the list is in the range `[0, 500]`.
+- `-100 <= Node.val <= 100`
+- `0 <= k <= 2 * 109`
+
+### Approach
+
+Brute Force Approach : We have to move the last element to first for each k.
+For each k, find the last element from the list. Move it to the first.
+
+Let’s take an example. 
+
+head = [1,2,3,4,5] k = 2000000000
+
+If we see a brute force approach, it will take O(5*2000000000) which is not a good time complexity when we can optimize it.
+
+We can see that for every k which is multiple of the length of the list, we get back the original list. Try to operate brute force on any linked list for k as a multiple of the length of the list.
+
+This gives us a hint that for k greater than the length of the list, we have to rotate the list for k%length of the list. This reduces our time complexity.
+
+Steps to the algorithm:-
+
+1. Calculate the length of the list.
+2. Connect the last node to the first node, converting it to a circular linked list.
+3. Iterate to cut the link of the last node and start a node of k%length of the list rotated list.
+
+### Solution Code
+
+#### Python
+
+```
+class Node:
+    def __init__(self, val):
+        self.val = val
+        self.next = None
+
+# utility function to insert node at the end of the linked list
+def insertNode(head, val):
+    newNode = Node(val)
+    if head == None:
+        head = newNode
+        return head
+    temp = head
+    while temp.next != None:
+        temp = temp.next
+    temp.next = newNode
+    return head
+
+# utility function to rotate list by k times
+def rotateRight(head, k):
+    if head == None or head.next == None or k == 0:
+        return head
+    # calculating length
+    temp = head
+    length = 1
+    while temp.next != None:
+        length += 1
+        temp = temp.next
+    # link last node to first node
+    temp.next = head
+    k = k % length  # when k is more than length of list
+    end = length - k  # to get end of the list
+    while end:
+        temp = temp.next
+        end -= 1
+    # breaking last node link and pointing to NULL
+    head = temp.next
+    temp.next = None
+
+    return head
+
+# utility function to print list
+def printList(head):
+    while head.next != None:
+        print(head.val, end='->')
+        head = head.next
+    print(head.val)
+    return
+
+if __name__ == '__main__':
+    head = None
+    # inserting Node
+    head = insertNode(head, 1)
+    head = insertNode(head, 2)
+    head = insertNode(head, 3)
+    head = insertNode(head, 4)
+    head = insertNode(head, 5)
+
+    print("Original list: ", end='')
+    printList(head)
+
+    k = 2
+    # calling function for rotating right of the nodes by k times
+    newHead = rotateRight(head, k)
+
+    print("After", k, "iterations: ", end='')
+    printList(newHead)  # list after rotating nodes
+```
+
+#### Java
+```
+//utility function to insert node at the end of the list
+static Node insertNode(Node head,int val) {
+    Node newNode = new Node(val);
+    if(head == null) {
+        head = newNode;
+        return head;
+    }
+    Node temp = head;
+    while(temp.next != null) temp = temp.next;
+
+    temp.next = newNode;
+    return head;
+}
+//utility function to rotate list by k times
+static Node rotateRight(Node head,int k) {
+    if(head == null||head.next == null||k == 0) return head;
+    //calculating length
+    Node temp = head;
+    int length = 1;
+    while(temp.next != null) {
+        ++length;
+        temp = temp.next;
+    }
+    //link last node to first node
+    temp.next = head;
+    k = k%length; //when k is more than length of list
+    int end = length-k; //to get end of the list
+    while(end--!=0) temp = temp.next;
+    //breaking last node link and pointing to NULL
+    head = temp.next;
+    temp.next = null;
+
+    return head;
+}
+
+//utility function to print list
+static void printList(Node head) {
+    while(head.next != null) {
+        System.out.print(head.num+"->");
+        head = head.next;
+    } 
+    System.out.println(head.num);
+
+}
+```
+
+#### C++
+```
+//utility function to insert node at the end of the list
+void insertNode(node* &head,int val) {
+    node* newNode = new node(val);
+    if(head == NULL) {
+        head = newNode;
+        return;
+    }
+    node* temp = head;
+    while(temp->next != NULL) temp = temp->next;
+
+    temp->next = newNode;
+    return;
+}
+//utility function to rotate list by k times
+node* rotateRight(node* head,int k) {
+    if(head == NULL||head->next == NULL||k == 0) return head;
+    //calculating length
+    node* temp = head;
+    int length = 1;
+    while(temp->next != NULL) {
+        ++length;
+        temp = temp->next;
+    }
+    //link last node to first node
+    temp->next = head;
+    k = k%length; //when k is more than length of list
+    int end = length-k; //to get end of the list
+    while(end--) temp = temp->next;
+    //breaking last node link and pointing to NULL
+    head = temp->next;
+    temp->next = NULL;
+
+    return head;
+}
+
+//utility function to print list
+void printList(node* head) {
+    while(head->next != NULL) {
+        cout<<head->num<<"->";
+        head = head->next;
+    } 
+    cout<<head->num<<endl;
+    return;
+}
+```
+
+## Complexity Analysis :
+
+- Time complexity: O(length of list) + O(length of list - (length of list%k))
+
+- Space complexity: O(1)