Learn and practice the In-place Reversal of a Linked List, Stacks, and Matrices patterns to solve coding interview problems.

Educative-77 in C++: Accelerate Your Coding Interview Prep

Interviewers want to be sure that the candidate is comfortable manipulating linked lists, arrays, and data structures based on them, such as stacks and matrices. This module will provide the practice you need to review the concepts and techniques needed to solve common problems involving these data structures.

Basic Data Structures

# Statement

The task is to reverse the nodes in groups of $k$ in a given linked list, where $k$ is a positive integer, and at most the length of the linked list. If any remaining nodes are not part of a group of $k$, they should remain in their original order. 

It is not allowed to change the values of the nodes in the linked list. Only the order of the nodes can be modified.

> **Note:** Use only  $O(1)$ extra memory space.

**Constraints:**

Let `n` be the number of nodes in a linked list. 

* $1 \leq$ `k` $\leq$ `n` $\leq 500$
* $0 \leq$ `Node.value` $\leq 1000$

# Examples


# Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:


head → 1 → 0 → 7 → 8 → 0 → 6 → 3 → 5 → 2 → 8 → 7 → 5 → 4 → 9 → NULL



head → 7 → 0 → 1 → 6 → 0 → 8 → 2 → 5 → 3 → 5 → 7 → 8 → 4 → 9 → NULL



head → 6 → 0 → 8 → 7 → 0 → 1 → 5 → 7 → 8 → 2 → 5 → 3 → 4 → 9 → NULL


head → 8 → 0 → 0 → 1 → 6 → 7 → 8 → 7 → 5 → 3 → 5 → 9 → 4 → 2 → NULL


What is the output if the following head of the linked list and value of $k$ are given as input?

head → 8 → 0 → 6 → 1 → 0 → 7 → 8 → 7 → 5 → 3 → 5 → 2 → 4 → 9 → NULL

k = 3


What is the output if the following head of the linked list and value of <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>k</mi></mrow><annotation encoding="application/x-tex">k</annotation></semantics></math>k are given as input?
head → 8 → 0 → 6 → 1 → 0 → 7 → 8 → 7 → 5 → 3 → 5 → 2 → 4 → 9 → NULL
k = 3

head → 8 → 9 → 9 → 0 → 0 → 6 → 6 → 7 → 7 → 8 → NULL

head → 8 → 7 → 7 → 6 → 6 → 0 → 0 → 9 → 9 → 8 → NULL

head → 8 → 7 → 7 → 6 → 6 → 8 → 9 → 9 → 0 → 0 → NULL

head → 0 → 0 → 9 → 9 → 8 → 8 → 7 → 7 → 6 → 6 → NULL

What is the output if the following head of the linked list and value of k are given as input?

 head → 6 → 6 → 7 → 7 → 8 → 8 → 9 → 9 → 0 → 0 → NULL

k = 5

What is the output if the following head of the linked list and value of k are given as input?
head → 6 → 6 → 7 → 7 → 8 → 8 → 9 → 9 → 0 → 0 → NULL
k = 5

head → 3 → 0 → 6 → 7 → 3 → 4 → 7 → 1 → 9 → 0 → 6 → 3 → 2 → 5 → 7 → 8 → 7 → 1 → 4 → 7 → 8 → 0 → NULL

head → 7 → 6 → 0 → 3 → 1 → 7 → 4 → 3 → 3 → 6 → 0 → 9 → 8 → 7 → 5 → 2 → 7 → 4 → 1 → 7 → 8 → 0 → NULL

head → 3 → 4 → 0 → 3 → 1 → 7 → 5 → 2 → 7 → 6 → 0 → 9 → 8 → 7 → 1 → 7 → 3 → 6 → 0 → 8 → 4 → 7 → NULL


head → 1 → 7 → 4 → 3 → 7 → 6 → 0 → 3 → 8 → 7 → 5 → 2 → 3 → 6 → 0 → 9 → 7 → 4 → 1 → 7 → 8 → 0 → NULL

What is the output if the following head of the linked list and value of k are given as input?

head → 3 → 4 → 7 → 1 → 3 → 0 → 6 → 7 → 2 → 5 → 7 → 8 → 9 → 0 → 6 → 3 → 7 → 1 → 4 → 7 → 8 → 0 → NULL

k = 4

What is the output if the following head of the linked list and value of k are given as input?
head → 3 → 4 → 7 → 1 → 3 → 0 → 6 → 7 → 2 → 5 → 7 → 8 → 9 → 0 → 6 → 3 → 7 → 1 → 4 → 7 → 8 → 0 → NULL
k = 4

head → 2 → 0 → 6 → 7 → 3 → 6 → 9 → 1 → 5 → 4 → 8 → NULL

head → 0 → 2 → 7 → 6 → 6 → 3 → 1 → 9 → 4 → 5 → 8 → NULL

head → 6 → 2 → 7 → 9 → 0 → 3 → 1 → 5 → 6 → 8 → 4 → NULL

head → 7 → 6 → 0 → 2 → 1 → 9 → 6 → 3 → 4 → 5 → 8 → NULL

What is the output if the following head of the linked list and value of k are given as input?

head → 6 → 7 → 2 → 0 → 9 → 1 → 3 → 6 → 5 → 4 → 8 → NULL

k = 2

What is the output if the following head of the linked list and value of k are given as input?
head → 6 → 7 → 2 → 0 → 9 → 1 → 3 → 6 → 5 → 4 → 8 → NULL
k = 2

Reverse Nodes in k-Group

Reverse Nodes in k-Group

# Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

# Try it yourself

Implement your solution in the following coding playground.

#include <vector>

std::vector<ListNode*> ReverseLinkedList(ListNode* head, int k){

	ListNode* current = head; 
	ListNode* previous = nullptr; 
 ListNode* next = nullptr; 
	
	for (int i = 0; i < k; i++){
	 next = current->next;
	 current->next = previous;
	 previous = current;
	 current = next;
	}
	
	return {previous, current};
}

from linked_list_node import LinkedListNode


# Template for the linked list
class LinkedList:
    # __init__ will be used to make a LinkedList type object.
    def __init__(self):
        self.head = None
    
    # insert_node_at_head method will insert a LinkedListNode at 
    # head of a linked list.
    def insert_node_at_head(self, node):
        if self.head:
            node.next = self.head
            self.head = node
        else:
            self.head = node
    
    # create_linked_list method will create the linked list using the
    # given integer array with the help of InsertAthead method. 
    def create_linked_list(self, lst):
        for x in reversed(lst):
            new_node = LinkedListNode(x)
            self.insert_node_at_head(new_node)
    
    # __str__(self) method will display the elements of linked list.
    def __str__(self):
        result = ""
        temp = self.head
        while temp:
            result += str(temp.data)
            temp = temp.next
            if temp:
                result += ", "
        result += ""
        return result


# Template for linked list node class

class LinkedListNode:
    # __init__ will be used to make a LinkedListNode type object.
    def __init__(self, data, next=None):
        self.data = data
        self.next = next


# Template for reversing a linked list

def reverse_linked_list(head):
	prev, curr = None, head
	while curr:
		nxt = curr.next
		curr.next = prev
		prev = curr
		curr = nxt
	return prev


# Template for traversing a linked list

def traverse_linked_list(head):
    current, nxt = head, None
    while current:
      nxt = current.next
      current = nxt



# Quick Fix\nYou are Edward, a Senior Software Engineer who specializes in coding interview patterns, data structures and algorithms, and is great at debugging issues in code.\n\n- Edward, please keep in mind that you do not have to print the data of the placeholders. Also, do not mention anything about yourself.\n\nEdward, you are given the following data in the placeholders:\n- Function name: `ReverseKGroups`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `C++`\n\n- Edward, your task is to understand the type of error that is occurring in the coding language provided.\n- Once you've understood the error as per the error’s description, and why the test case failed, you will suggest how the candidate can improve their solution code to fix the error.\n- Edward, you will not give the corrected code, just a hint on what the error tells, and how to fix it.\n- Edward, please limit your response strictly to 100 words. No more words than the limit.

# Review Code\nEdward, your current task is to review the `ReverseKGroups` function(s) and the other function(s) invoked in it/them.\n\n-Take your time and review the candidate's solution code.\n- Do not review any extra/helper files or functions not invoked in the solution function `ReverseKGroups`.\n- Do not review the code of data structure objects instantiated like a binary tree, linked list, trie, union find, etc., in the solution functions. Just review the solution/algorithm function of the problem. \n- Also, if the solution code doesn’t use the coding pattern stated above, then please encourage the candidate to reattempt the problem using that pattern corectly.

# Review time complexity\n- Please provide a brief time complexity analysis of the function(s) `ReverseKGroups`. \n- While calculating time complexity, include the time complexity of built-in C++ functions invoked by the code. \n- The answer should be to the point in approximately 60 words.

# Review space complexity\n- Please provide a brief space complexity analysis of the function(s) `ReverseKGroups`. \n- While calculating space complexity, include the space complexity of built-in C++ functions invoked by the code. \n- Do not include the space used for the input parameters and the space used for just returning an output (not used as a scratch space). \n- The answer should be to the point in approximately 60 words.

def evaluate_results(self) -> object:
        """
        self.__inputs is the list of inputs that are passed as input to function
        self.__actual_output is a list of outputs that is produced by the learner's code
        return : self.__edu__result object
        """
        # Template for linked list node class

        class LinkedListNode:
            # __init__ will be used to make a LinkedListNode type object.
            def __init__(self, data, next=None):
                self.data = data
                self.next = next

        # Template for the linked list
        class LinkedList:
            # __init__ will be used to make a LinkedList type object.
            def __init__(self):
                self.head = None
            
            # insert_node_at_head method will insert a LinkedListNode at 
            # head of a linked list.
            def insert_node_at_head(self, node):
                if self.head:
                    node.next = self.head
                    self.head = node
                else:
                    self.head = node
            
            # create_linked_list method will create the linked list using the
            # given integer array with the help of InsertAthead method. 
            def create_linked_list(self, lst):
                for x in reversed(lst):
                    new_node = LinkedListNode(x)
                    self.insert_node_at_head(new_node)
            
            # __str__(self) method will display the elements of linked list.
            def __str__(self):
                result = ""
                temp = self.head
                while temp:
                    result += str(temp.data)
                    temp = temp.next
                    if temp:
                        result += ", "
                result += ""
                return result

        def reverse_linked_list_1(head, k):
            previous, current, next = None, head, None
            for _ in range(k):
                # temporarily store the next node
                next = current.next
                # reverse the current node
                current.next = previous
                # before we move to the next node, point previous to the
                # current node
                previous = current
                # move to the next node 
                current = next
            return previous, current

        def reverse_k_groups_1(head, k):

            dummy = LinkedListNode(0)
            dummy.next = head
            ptr = dummy
        

            while(ptr != None):

                tracker = ptr

                for i in range(k):

                    if tracker == None:
                        break
            
                    tracker = tracker.next

                if tracker == None:
                    break
            
                previous, current = reverse_linked_list_1(ptr.next, k)

                last_node_of_reversed_group = ptr.next
                last_node_of_reversed_group.next = current
                ptr.next = previous
                ptr = last_node_of_reversed_group

            return dummy.next

        # multiple outputs will be added at each index respectively for a single test_case
        input_0 = LinkedList()
        input_0.create_linked_list(self.__inputs[0])
        expected = reverse_k_groups_1(input_0.head, self.__inputs[1])

        actual = self.__actual_outputs[0]
        # print(actual)
        expected_list_values = []
        current_1 = expected
        while current_1:
            expected_list_values.append(current_1.data)
            current_1 = current_1.next

        
        actual_list_values = []

        expected_outputs = [] 
        expected_outputs.append(expected_list_values)

        if expected_list_values == actual:
            self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
        else:
            self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

        return self.__edu_result

#include"LinkedListReversal.cpp"
// Definition for a Linked List node
// class ListNode {
// public:
//     int val;
//     ListNode* next;

//     // Constructor
//     ListNode(int val = 0, ListNode* next = nullptr);
// };

ListNode *ReverseKGroups(ListNode *head, int k) {
  
    // Replace this placeholder return statement with your code
    return nullptr;
}

reverse_nodes_in_k_group.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : [1, 2, 3, 4, 5]
    - 2 : 2
    output: 
    - 1 : [2,1,4,3,5]
  - name: "testcase 2"
    inputs:
    - 1: [3, 4, 5, 6, 2, 8, 7, 7]
    - 2 : 3
    output:
    - 1 : [5,4,3,8,2,6,7,7]
  - name: "testcase 3"
    inputs:
    - 1: [1, 2, 3, 4, 5]
    - 2 : 1
    output:
    - 1 : [1,2,3,4,5]
  - name: "testcase 4"
    inputs:
    - 1: [1, 2, 3, 4, 5, 6, 7]
    - 2 : 3
    output:
    - 1 : [3,2,1,6,5,4,7]
  - name: "testcase 5"
    inputs:
    - 1: [1, 2, 3, 4, 5, 6, 7]
    - 2 : 7
    output:
    - 1 : [7,6,5,4,3,2,1]

def sum(input_1, input_2):
    return input_1+input_2

def validate_inputs(self) -> None:
 """
 self.__inputs is the list of inputs that are passed as input to function
 store the result using the self.store_result(error_msg, status)
 return : void
 """
 def is_valid_1d_array(arr):
 if not isinstance(arr, list):
 return False # Not a valid 1D array if it's not a list
 
 return all(isinstance(elem, int) and not isinstance(elem, bool) for elem in arr) 
 inp_1 = self.__inputs[0]
 inp_2 =self.__inputs[1]
 # inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if not is_valid_1d_array(inp_1):
 self.store_result(("Invalid test case."), ValidationFunction.FAIL) 
 elif not (1 <= len(inp_1) <= 500):
 self.store_result(("The number of nodes in the linked list should be in the range [1, 500] but got " + str(len(inp_1)) + "."), ValidationFunction.FAIL)
 elif not(1 <= inp_2 <= len(inp_1)):
 self.store_result(("Expected 'k' to have value from 1 to " + str(len(inp_1)) + " only."), ValidationFunction.FAIL)
 
 elif not all(0 <= num <= 1000 for num in inp_1):
 self.store_result ("Linked list must consist of values from 0 to 1000 only", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Reverse Nodes in k-Group problem.

In-Place Manipulation of a Linked List

Stacks

Matrices

Conclusion

Reverse Nodes in k-Group

Statement

Examples

Understand the problem

Figure it out!

Try it yourself