Valid Palindrome Two Pointer in DSA C - Execution Trace

Concept Flow - Valid Palindrome Two Pointer

Start: left=0, right=length-1

↓

Check if left < right?

No→Done: Palindrome

Yes↓

Is s[left

Yes↓

Is s[right

Yes↓

Compare s[left

↓

left++, right--

Two pointers start at the string ends, move inward skipping non-alphanumeric characters, comparing characters case-insensitively until pointers cross or mismatch.

Execution Sample

DSA C

bool isPalindrome(char * s) {
    int left = 0, right = strlen(s) - 1;
    while (left < right) {
        if (!isalnum(s[left])) left++;
        else if (!isalnum(s[right])) right--;
        else if (tolower(s[left]) != tolower(s[right])) return false;
        else { left++; right--; }
    }
    return true;
}

Checks if a string is a palindrome ignoring non-alphanumeric characters and case using two pointers.

Execution Table

Step	Operation	left	right	Characters Compared	Action	Visual State
1	Initialize pointers	0	29	-	Start	"A man, a plan, a canal: Panama"
2	Check s[left] alnum	0	29	A vs a	Compare (equal)	"A man, a plan, a canal: Panama"
3	Move pointers	1	28	-	left++, right--	"A man, a plan, a canal: Panama"
4	Check s[left] alnum	1	28	vs m	left not alnum, left++	"A man, a plan, a canal: Panama"
5	Check s[left] alnum	2	28	m vs m	Compare (equal)	"A man, a plan, a canal: Panama"
6	Move pointers	3	27	-	left++, right--	"A man, a plan, a canal: Panama"
7	Check s[left] alnum	3	27	a vs a	Compare (equal)	"A man, a plan, a canal: Panama"
8	Move pointers	4	26	-	left++, right--	"A man, a plan, a canal: Panama"
9	Check s[left] alnum	4	26	n vs n	Compare (equal)	"A man, a plan, a canal: Panama"
10	Move pointers	5	25	-	left++, right--	"A man, a plan, a canal: Panama"
11	Check s[left] alnum	5	25	, vs a	left not alnum, left++	"A man, a plan, a canal: Panama"
12	Check s[left] alnum	6	25	vs a	left not alnum, left++	"A man, a plan, a canal: Panama"
13	Check s[left] alnum	7	25	a vs a	Compare (equal)	"A man, a plan, a canal: Panama"
14	Move pointers	8	24	-	left++, right--	"A man, a plan, a canal: Panama"
15	Check s[left] alnum	8	24	vs P	left not alnum, left++	"A man, a plan, a canal: Panama"
16	Check s[left] alnum	9	24	p vs P	Compare (equal)	"A man, a plan, a canal: Panama"
17	Move pointers	10	23	-	left++, right--	"A man, a plan, a canal: Panama"
18	Check s[left] alnum	10	23	l vs	right not alnum, right--	"A man, a plan, a canal: Panama"
19	Check s[left] alnum	10	22	l vs :	right not alnum, right--	"A man, a plan, a canal: Panama"
20	Check s[left] alnum	10	21	l vs l	Compare (equal)	"A man, a plan, a canal: Panama"
21	Move pointers	11	20	-	left++, right--	"A man, a plan, a canal: Panama"
22	Check s[left] alnum	11	20	a vs a	Compare (equal)	"A man, a plan, a canal: Panama"
23	Move pointers	12	19	-	left++, right--	"A man, a plan, a canal: Panama"
24	Check s[left] alnum	12	19	n vs n	Compare (equal)	"A man, a plan, a canal: Panama"
25	Move pointers	13	18	-	left++, right--	"A man, a plan, a canal: Panama"
26	Check s[left] alnum	13	18	, vs a	left not alnum, left++	"A man, a plan, a canal: Panama"

💡 After step 26 (left=14, right=18), continue similarly: skip left ' ' at 14 to 15, compare 'a' vs 'a' at 15/18, move to 16/17; skip left ' ' at 16 to 17; left == right =17, all pairs matched, palindrome.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	After 10	Final
left	0	1	2	3	4	5	7	8	9	10	10	17
right	29	28	28	27	26	25	25	24	24	23	21	17

Key Moments - 3 Insights

Why do we skip characters that are not alphanumeric?

Because the palindrome check ignores spaces, punctuation, and symbols. See steps 4, 11, 12, 15, and 26 in the execution_table where pointers move without comparison.

Why do we compare characters in lowercase?

To make the comparison case-insensitive. For example, 'A' and 'a' are considered equal as shown in step 2 and 'p' and 'P' in step 16.

When does the loop stop?

The loop stops when left pointer is no longer less than right pointer, meaning all pairs have been checked. This occurs when pointers meet or cross after final skips (left=17, right=17).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'left' after step 5?

A 2

B 3

C 1

D 5

💡 Hint

Check the 'left' column in execution_table row with Step 5.

At which step does the right pointer first move without comparison because of a non-alphanumeric character?

A Step 4

B Step 11

C Step 18

D Step 25

💡 Hint

Look for steps where 'Action' says 'right not alnum, right--' in execution_table.

If the input string was "race a car", at which step would the function return false?

A When comparing 'e' and 'a'

B When comparing 'r' and 'r'

C When comparing 'c' and 'c'

D When comparing 'a' and 'a'

💡 Hint

Think about which characters mismatch in the palindrome check and when the function returns false.

Concept Snapshot

Two pointers start at string ends
Skip non-alphanumeric chars
Compare characters case-insensitive
Move pointers inward if equal
Stop when pointers cross or mismatch

Full Transcript

This visualization shows how to check if a string is a palindrome using two pointers. We start with one pointer at the beginning and one at the end. We skip characters that are not letters or numbers. We compare characters ignoring case. If they match, we move both pointers inward. If they don't match, we stop and say it's not a palindrome. The process continues until the pointers cross, meaning the string is a palindrome.