Valid Palindrome Two Pointer in DSA Python - Execution Trace

Concept Flow - Valid Palindrome Two Pointer

Start with left=0, right=len(s)-1

↓

Check if s[left

Yes↓

Check if s[right

Yes↓

Compare s[left

Not Equal↓

Return False

↓

Repeat until left >= right

↓

Return True

Two pointers start at the string ends, skip non-alphanumeric chars, compare characters, move inward until pointers cross or mismatch.

Execution Sample

DSA Python

def is_palindrome(s):
    left, right = 0, len(s) - 1
    while left < right:
        if not s[left].isalnum():
            left += 1
        elif not s[right].isalnum():
            right -= 1
        elif s[left].lower() != s[right].lower():
            return False
        else:
            left += 1
            right -= 1
    return True

Checks if string s is a palindrome ignoring non-alphanumeric characters and case.

Execution Table

Step	Operation	left	right	s[left]	s[right]	Action	Result
1	Initialize pointers	0	14	A	a	Compare 'A' and 'a'	Equal (case-insensitive), move inward
2	Move pointers	1	13		r	s[left] is space, skip left	left=2
3	Skip non-alnum left	2	13	m	r	Compare 'm' and 'r'	Not equal, return False
4	Exit	-	-	-	-	-	Palindrome check failed at step 3

💡 Mismatch found at step 3, characters 'm' and 'r' differ ignoring case

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
left	0	1	2	-	-
right	14	13	13	-	-
s[left]	A		m	-	-
s[right]	a	r	r	-	-

Key Moments - 3 Insights

Why do we skip characters that are not alphanumeric?

Because the palindrome check ignores spaces and punctuation. See execution_table step 2 where left pointer skips a space.

Why do we compare characters in lowercase?

To make the check case-insensitive. In step 1, 'A' and 'a' are considered equal after lowering.

What happens when characters don't match?

The function returns False immediately, as shown in step 3 where 'm' and 'r' differ.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'left' after step 2?

A 2

B 1

C 0

D 3

💡 Hint

Check the 'left' column in execution_table row for step 2

At which step does the function decide the string is not a palindrome?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Result' column where it returns False

If the input string had no spaces, how would step 2 change?

A left pointer would not skip and stay same

B right pointer would skip instead

C left pointer would skip more characters

D Function would return True immediately

💡 Hint

Refer to step 2 where left pointer skips space; no spaces means no skip

Concept Snapshot

Two pointers start at string ends
Skip non-alphanumeric chars
Compare lowercase chars
If mismatch, return False
If pointers cross, return True

Full Transcript

This concept uses two pointers starting at the beginning and end of the string. Each pointer moves inward, skipping characters that are not letters or numbers. Characters are compared in lowercase to ignore case differences. If any pair does not match, the function returns False immediately. If the pointers cross without mismatches, the string is a palindrome and returns True.