Valid Palindrome Two Pointer in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the two-pointer technique in checking a palindrome?

Use two pointers starting at the beginning and end of the string, moving towards the center, comparing characters to check if they are the same.

beginner

Why do we skip non-alphanumeric characters in the Valid Palindrome problem?

Because only letters and numbers matter for palindrome checking; spaces, punctuation, and symbols are ignored.

beginner

In the two-pointer approach, what happens if characters at pointers do not match?

We conclude the string is not a palindrome and stop checking further.

beginner

How do you handle case differences when checking for palindrome?

Convert both characters to the same case (usually lowercase) before comparing.

intermediate

What is the time complexity of the two-pointer palindrome check?

O(n), where n is the length of the string, because each character is checked at most once.

What do the two pointers represent in the Valid Palindrome problem?

A Start and end indices of the string

B Middle characters of the string

C Random positions in the string

D Only the first character

The two pointers start at the beginning and end of the string and move towards the center.

Which characters are ignored when checking for a valid palindrome?

A Letters only

B Alphanumeric characters

C Spaces and punctuation

D Numbers only

Spaces, punctuation, and symbols are ignored; only letters and numbers are considered.

What should you do if characters at the two pointers do not match?

A Return false immediately

B Move pointers inward and continue

C Skip the characters

D Convert characters to uppercase

If characters differ, the string is not a palindrome, so return false.

How do you handle uppercase and lowercase letters in palindrome checking?

A Convert both to uppercase

B Compare as is

C Only compare lowercase letters

D Ignore case by converting both to lowercase

Converting both characters to lowercase ensures case-insensitive comparison.

What is the time complexity of the two-pointer palindrome check?

A O(n^2)

B O(n)

C O(log n)

D O(1)

Each character is checked at most once, so the time complexity is linear, O(n).

Explain how the two-pointer technique works to check if a string is a valid palindrome.

Think about comparing characters from both ends moving towards the center.

Describe how to handle characters that are not letters or numbers when checking for a palindrome.

Consider what to do with spaces and punctuation.

Practice

1. What does the two-pointer technique check when verifying if a string is a palindrome?

easy

A. It compares characters from the start and end moving towards the center

B. It counts the number of vowels in the string

C. It reverses the string and compares with the original

D. It sorts the characters and checks if sorted equals original

Solution

1. Step 1: Understand two-pointer approach

   The two-pointer technique uses two indexes: one at the start and one at the end of the string.

2. Step 2: Compare characters moving inward

   It compares characters at these pointers and moves them towards the center to check if they match.

3. Final Answer:

   It compares characters from the start and end moving towards the center -> Option A

4. Quick Check:

   Two-pointer = compare start/end chars [OK]

Hint: Check characters from both ends moving inward [OK]

Common Mistakes:

• Thinking it counts vowels instead of comparing characters
• Assuming it reverses the string explicitly
• Confusing sorting with palindrome check

2. Which of the following Go code snippets correctly initializes two pointers for a palindrome check on string s?

easy

A. left := len(s); right := 0

B. left := 1; right := len(s)

C. left := -1; right := len(s) + 1

D. left := 0; right := len(s) - 1

Solution

1. Step 1: Identify valid pointer initialization

   For a string, the first character index is 0 and last is length minus 1.

2. Step 2: Check each option

   left := 0; right := len(s) - 1 correctly sets left to 0 and right to len(s)-1, valid for two-pointer palindrome check.

3. Final Answer:

   left := 0; right := len(s) - 1 -> Option D

4. Quick Check:

   Start=0, End=len(s)-1 [OK]

Hint: Start left at 0 and right at len(s)-1 [OK]

Common Mistakes:

• Starting left at 1 instead of 0
• Setting right to len(s) which is out of range
• Using negative or invalid indices

3. What will be the output of the following Go code?

func isPalindrome(s string) bool {
    left, right := 0, len(s)-1
    for left < right {
        if s[left] != s[right] {
            return false
        }
        left++
        right--
    }
    return true
}

fmt.Println(isPalindrome("racecar"))
fmt.Println(isPalindrome("hello"))

medium

A. true\nfalse

B. false\ntrue

C. true\ntrue

D. false\nfalse

Solution

1. Step 1: Trace "racecar" input

   Characters at start and end match moving inward, so function returns true.

2. Step 2: Trace "hello" input

   Characters 'h' and 'o' differ at first check, so function returns false.

3. Final Answer:

   true\nfalse -> Option A

4. Quick Check:

   "racecar" palindrome = true, "hello" not = false [OK]

Hint: Check first and last chars; mismatch means false [OK]

Common Mistakes:

• Assuming "hello" is palindrome
• Confusing return values order
• Ignoring early return on mismatch

4. Identify the bug in this Go function that checks palindrome:

func isPalindrome(s string) bool {
    left, right := 0, len(s)
    for left < right {
        if s[left] != s[right] {
            return false
        }
        left++
        right--
    }
    return true
}

medium

A. left should start at 1, not 0

B. The loop condition should be left <= right

C. right should be initialized to len(s)-1, not len(s)

D. The function should return false at the end

Solution

1. Step 1: Check pointer initialization

   right is set to len(s), which is out of string index range (0 to len(s)-1).

2. Step 2: Correct initialization

   right should be len(s)-1 to point to last character correctly.

3. Final Answer:

   right should be initialized to len(s)-1, not len(s) -> Option C

4. Quick Check:

   Index out of range error fixed by right = len(s)-1 [OK]

Hint: Set right pointer to len(s)-1 to avoid index error [OK]

Common Mistakes:

• Using len(s) as index causes runtime error
• Changing left pointer start incorrectly
• Misunderstanding loop condition

5. Given the string s = "A man, a plan, a canal: Panama", which modification to the two-pointer palindrome check is needed to correctly identify it as a palindrome?

hard

A. Reverse the string and compare directly

B. Ignore non-alphanumeric characters and compare case-insensitively

C. Only compare vowels in the string

D. Convert string to uppercase and compare characters directly

Solution

1. Step 1: Understand input complexity

   The string contains spaces, commas, and mixed cases which should be ignored for palindrome check.

2. Step 2: Modify two-pointer logic

   Skip non-alphanumeric characters and compare characters in lowercase to handle case differences.

3. Final Answer:

   Ignore non-alphanumeric characters and compare case-insensitively -> Option B

4. Quick Check:

   Clean and lowercase string before two-pointer check [OK]

Hint: Skip symbols and compare lowercase chars [OK]

Common Mistakes:

• Not ignoring spaces and punctuation
• Comparing case-sensitively causing false negatives
• Only converting case without skipping symbols