Palindrome Detection in DSA C - Execution Trace

Concept Flow - Palindrome Detection

Start

↓

Set left = 0, right = length-1

↓

Compare s[left

↓

Return False

Yes↓

Increment left, Decrement right

↓

Check if left >= right

↓

Return True

No↓

left

Start with two pointers at the string ends, compare characters moving inward until mismatch or pointers cross.

Execution Sample

DSA C

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

Checks if a string is a palindrome by comparing characters from both ends moving inward.

Execution Table

Step	Operation	left	right	Characters Compared	Match?	Action	Visual State
1	Initialize pointers	0	6	-	-	Set left=0, right=6	"r" "a" "c" "e" "c" "a" "r"
2	Compare s[left] and s[right]	0	6	r vs r	Yes	left++, right--	"r" "a" "c" "e" "c" "a" "r"
3	Compare s[left] and s[right]	1	5	a vs a	Yes	left++, right--	"r" "a" "c" "e" "c" "a" "r"
4	Compare s[left] and s[right]	2	4	c vs c	Yes	left++, right--	"r" "a" "c" "e" "c" "a" "r"
5	Compare s[left] and s[right]	3	3	e vs e	Yes	left >= right, return True	"r" "a" "c" "e" "c" "a" "r"
6	End	-	-	-	-	Palindrome detected	"r" "a" "c" "e" "c" "a" "r"

💡 Pointers crossed at step 5, all characters matched, string is palindrome

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
left	0	1	2	3	3	3
right	6	5	4	3	3	3

Key Moments - 3 Insights

Why do we stop comparing when left >= right?

Because when left meets or passes right, all pairs have been checked. See execution_table step 5 where left=3 and right=3 means middle reached.

What if characters at left and right don't match?

We immediately return false as shown in execution_table step 2-4 'Match?' column. If any mismatch occurs, palindrome check fails.

Why do we increment left and decrement right after each successful comparison?

To move inward and compare the next pair of characters, as shown in execution_table steps 2-4 'Action' column.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what are the values of left and right?

A left=2, right=4

B left=3, right=3

C left=1, right=5

D left=0, right=6

💡 Hint

Check the 'left' and 'right' columns in execution_table row with Step=3

At which step does the function decide the string is a palindrome?

A Step 5

B Step 2

C Step 4

D Step 6

💡 Hint

Look at the 'Action' column where it returns True when left >= right

If the string was "racecarx", what would happen at step 2?

A Characters match, continue

B Characters don't match, return false

C Pointers cross, return true

D Error due to string length

💡 Hint

Refer to 'Match?' column in execution_table for mismatch behavior

Concept Snapshot

Palindrome Detection:
- Use two pointers: left at start, right at end
- Compare characters s[left] and s[right]
- If mismatch, return false immediately
- If pointers cross or meet, return true
- Moves inward by left++ and right-- each step

Full Transcript

Palindrome detection uses two pointers starting at the ends of the string. We compare characters at these pointers. If they match, we move the pointers inward. If any pair does not match, the string is not a palindrome. When the pointers meet or cross, it means all pairs matched and the string is a palindrome.