Learn and practice the Two Pointers and Modified Binary Search patterns to solve coding interview problems.

Speedrun the Meta Coding Interview: Top Problems in JavaScript

This module groups together two of the most common patterns that may be used to solve problems where the input data is linear: Two Pointers and Modified Binary Search. Both the Two Pointers and the Modified Binary Search patterns derive their power from their ability to efficiently reduce the size of the search space, that is, both exemplify the divide and conquer approach to problem-solving. After completing this module, we will have the knowledge of how to use this set of related patterns to solve a diverse range of problems.

Two Pointers & Co.

What is the output for the following input?

s = "A man, a plan, a canal: Panama"

What is the output for the following input?
s = “A man, a plan, a canal: Panama”

What is the output for the following input?

s = "hello"

What is the output for the following input?
s = “hello”

What is the output for the following input?

s = "Was it a car or a cat I saw?"

What is the output for the following input?
s = “Was it a car or a cat I saw?”

What is the output for the following input?

s = "123421"

What is the output for the following input?
s = “123421”

What is the output for the following input?

s = "No lemon, no melon!"

What is the output for the following input?
s = “No lemon, no melon!”

Valid Palindrome

Valid Palindrome

As we only care about alphanumeric characters, can we first process the string to remove everything else and convert it all to lowercase? That will simplify our comparisons.

Instead of building a new reversed string, try using two pointers: one starting from the left and one from the right. Compare characters at both ends and move the pointers inward.

When should the comparison stop? Think about the stopping condition when the two pointers meet or cross each other.

# How to set up two pointers in an array

def two_pointers(array):
 left = 0
 right = len(array) - 1
 while left <= right:
 left = left + 1
 right = right -1

# 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: `isPalindrome`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `JavaScript`\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 `isPalindrome` 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 `isPalindrome`.\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) `isPalindrome`. \n- While calculating time complexity, include the time complexity of built-in JavaScript 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) `isPalindrome`. \n- While calculating space complexity, include the space complexity of built-in JavaScript 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
 """
 def is_palindrome(s):
 # Initialize two pointers: 
 # 'left' starts from the beginning, 'right' starts from the end
 left, right = 0, len(s) - 1
 
 # Loop until the two pointers meet or cross each other
 while left < right:
 
 # Move left pointer forward if it is pointing to a non-alphanumeric character
 while left < right and not s[left].isalnum():
 left += 1 # Skip to the next character
 
 # Move right pointer backward if it is pointing to a non-alphanumeric character
 while left < right and not s[right].isalnum():
 right -= 1 # Skip to the previous character
 
 # Compare characters (convert to lowercase for case-insensitive comparison)
 if s[left].lower() != s[right].lower():
 return False # Not a palindrome
 
 # Move both pointers inward to check the next pair of characters
 left += 1
 right -= 1
 
 # If the loop completes without mismatches, the string is a palindrome
 return True



 # multiple outputs will be added at each index respectively for a single test_case
 expected_outputs = [] 
 expected = is_palindrome(self.__inputs[0])
 actual = self.__actual_outputs[0]
 # store the 1st expected output that should be displayed to the user 
 expected_outputs.append(expected) 
 if expected == actual:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
 else:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

 return self.__edu_result

valid-palindrome-private-testcases.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1: "kaYak"
    output:
    - 1 : True
  - name: "testcase 2"
    inputs:
    - 1 : "hello"
    output: 
    - 1 : False
  - name: "testcase 3"
    inputs:
    - 1: "RaCEACAR"
    output:
    - 1: False
  - name: "testcase 4"
    inputs:
    - 1: "Madam, in Eden, Im Adam"
    output:
    - 1: True
  - name: "testcase 5"
    inputs:
    - 1: "@#$%^&*"
    output:
    - 1: True
    

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 the function.
 Store the result using the self.store_result(error_msg, status).
 Return: void
 """
 import re
 # Inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if len(self.__inputs[0]) < 1 or len(self.__inputs[0]) > 3000:
 self.store_result("Expected length of input 1 to be between 1 and 3000.", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Valid Palindrome problem.

Two Pointers

Modified Binary Search

Conclusion

Valid Palindrome

Statement

Examples

Understand the problem

Figure it out!

Try it yourself