Palindrome Detection in DSA Python - Execution Trace

Concept Flow - Palindrome Detection

Start with input string

↓

Set left pointer at start

↓

Set right pointer at end

↓

Compare characters at left and right

↓

Move left++ and right--

↓

Left >= Right?

↓

Repeat compare

↓

Return False

We use two pointers from start and end, compare characters moving inward until mismatch or pointers cross.

Execution Sample

DSA Python

def is_palindrome(s):
    left, right = 0, len(s) - 1
    while left < right:
        if s[left] != s[right]:
            return False
        left += 1
        right -= 1
    return True

Checks if a string reads the same forwards and backwards by comparing characters from both ends.

Execution Table

Step	Operation	Left Pointer	Right Pointer	Characters Compared	Match?	Action	Result So Far
1	Initialize pointers	0	6	-	-	Start comparing	Undecided
2	Compare s[0] and s[6]	0	6	'r' vs 'r'	Yes	Move pointers inward	Undecided
3	Update pointers	1	5	-	-	Continue	Undecided
4	Compare s[1] and s[5]	1	5	'a' vs 'a'	Yes	Move pointers inward	Undecided
5	Update pointers	2	4	-	-	Continue	Undecided
6	Compare s[2] and s[4]	2	4	'c' vs 'c'	Yes	Move pointers inward	Undecided
7	Update pointers	3	3	-	-	Pointers crossed or equal	Undecided
8	Check pointers	3	3	-	-	Left >= Right, return True	Palindrome confirmed

💡 Pointers crossed (left 3 >= right 3), all compared characters matched, string is palindrome

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
left	0	1	2	3	3	3
right	6	5	4	3	3	3
result	Undecided	Undecided	Undecided	Undecided	Palindrome confirmed	Palindrome confirmed

Key Moments - 3 Insights

Why do we stop comparing when left pointer is equal to or greater than right pointer?

Because when left >= right, all character pairs have been checked. See execution_table step 8 where pointers meet, confirming palindrome.

What happens if characters at left and right pointers do not match?

The function immediately returns False, ending the check early. This is shown in the execution_table where 'Match?' would be 'No' and action would be 'Return False'.

Why do we move left pointer forward and right pointer backward after each successful comparison?

To check the next inner pair of characters. This is shown in steps 3, 5, and 7 where pointers update to move inward.

Visual Quiz - 3 Questions

Look at the execution_table, what characters are compared at step 6?

A 'a' vs 'a'

B 'c' vs 'c'

C 'r' vs 'r'

D 'd' vs 'd'

💡 Hint

Check the 'Characters Compared' column at step 6 in execution_table.

At which step do the pointers cross or become equal, ending the loop?

A Step 8

B Step 4

C Step 6

D Step 2

💡 Hint

Look at the 'Action' and 'Left Pointer'/'Right Pointer' columns in execution_table for when left >= right.

If the string was 'racecar', what would be the result after step 8?

A Undecided

B Not a palindrome

C Palindrome confirmed

D Error

💡 Hint

Refer to the 'Result So Far' column at step 8 in execution_table.

Concept Snapshot

Palindrome Detection:
- Use two pointers: left at start, right at end
- Compare characters at left and right
- If mismatch, return False immediately
- Move pointers inward if match
- Stop when left >= right
- Return True if all pairs matched

Full Transcript

Palindrome detection checks if a string reads the same forwards and backwards. We start with two pointers: one at the beginning (left) and one at the end (right) of the string. We compare the characters at these pointers. If they match, we move the left pointer forward and the right pointer backward to check the next pair. If at any point the characters do not match, we return False immediately because the string is not a palindrome. We continue this process until the left pointer is equal to or greater than the right pointer, which means all character pairs have been checked and matched. At this point, we return True, confirming the string is a palindrome.