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

Educative-99 in Python: 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
Given the head of a singly linked list, reorder the list as if it were folded on itself. For example, if the list is represented as follows:


$L_{0}$ → $L_{1}$ → $L_{2}$  → … → $L_{n-2}$ → $L_{n-1}$ → $L_{n}$
​

This is how you’ll reorder it:

$L_{0}$ → $L_{n}$ → $L_{1}$ → $L_{n - 1}$ → $L_{2}$ → $L_{n - 2}$ → … 

You don’t need to modify the values in the list’s nodes; only the links between nodes need to be changed.

**Constraints:**

* The range of number of nodes in the list is $[1, 500]$.
 * $-5000 \leq$ `Node.value` $\leq 5000$

# 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:


What is the output if the following linked list is given as input?

4 → 2 → 7 → 8 → 9 → 0 → 2

What is the output if the following linked list is given as input?
4 → 2 → 7 → 8 → 9 → 0 → 2

What is the output if the following linked list is given as input?

0 → 6 → 7

What is the output if the following linked list is given as input?
0 → 6 → 7

What is the output if the following linked list is given as input?

9 → 5 → 8 → 2 → 1

What is the output if the following linked list is given as input?
9 → 5 → 8 → 2 → 1

6 → 0 → 6 → 7 → 0 → 7 → 8 → 9 → 8 → 9

0 → 6 → 0 → 7 → 9 → 7 → 9 → 8 → 8

8 → 8 → 9 → 7 → 9 → 7 → 0 → 6 → 0 → 6

6 → 0 → 6 → 0 → 7 → 9 → 7 → 9 → 8 → 8

What is the output if the following linked list is given as input?

6 → 6 → 7 → 7 → 8 → 8 → 9 → 9 → 0 → 0

What is the output if the following linked list is given as input?
6 → 6 → 7 → 7 → 8 → 8 → 9 → 9 → 0 → 0

Reorder List

Reorder List

# 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.

# 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: `reorder_list`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `Python`\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 `reorder_list` 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 `reorder_list`.\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) `reorder_list`. \n- While calculating time complexity, include the time complexity of built-in Python 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) `reorder_list`. \n- While calculating space complexity, include the space complexity of built-in Python 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 reorder_list(head):
            if not head:
                return head
            slow = fast = head
            
            while fast and fast.next:
                slow = slow.next
                fast = fast.next.next 
            prev, curr = None, slow

            while curr:
                curr.next, prev, curr = prev, curr, curr.next     
            first, second = head, prev

            while second.next:
                first.next, first = second, first.next
                second.next, second = first, second.next
            
            return head


        # 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 = reorder_list(input_0.head)

        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

# Definition for a Linked List node
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
from ds_v1.LinkedList.LinkedList import ListNode
            
def reorder_list(head):

    # Replace this placeholder return statement with your code
    
    return head


reorder-list.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : [1, 1, 2, 2, 3, -1, 10, 12]
    output: 
    - 1 : [1, 12, 1, 10, 2, -1, 2, 3]
  - name: "testcase 2"
    inputs:
    - 1: [10, 20, -22, 21, -12]
    output:
    - 1 : [10, -12, 20, 21, -22]
  - name: "testcase 3"
    inputs:
    - 1: [1,3,5,7,9,10,8,6,4,2]
    output:
    - 1 : [1,2,3,4,5,6,7,8,9,10]
  - name: "testcase 4"
    inputs:
    - 1: [1, 2, 3, 4, 5, 6]
    output:
    - 1 : [1, 6, 2, 5, 3, 4]
  - name: "testcase 5"
    inputs:
    - 1: [7, 0, 10, 13, 12, 19, 1, 3, 6, 7, 4, 2, 11]
    output:
    - 1 : [7, 11, 0, 2, 10, 4, 13, 7, 12, 6, 19, 3, 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 = self.__inputs[0]
 # inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if not is_valid_1d_array(inp):
 self.store_result(("Invalid test case."), ValidationFunction.FAIL)
 elif not (1 <= len(inp) <= 500):
 self.store_result(("The number of nodes in the linked list should be in the range [1, 500] but got " + str(len(inp)) + "."), ValidationFunction.FAIL)
 elif not all(-5000 <= num <= 5000 for num in inp):
 self.store_result ("Linked list must consist of values from -5000 to 5000 only", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

In-Place Manipulation of a Linked List

Stacks

Matrices

Conclusion

Reorder List

Statement

Examples

Understand the problem

Figure it out!

Try it yourself