Palindrome Detection in DSA Rust - 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

↓

Not a palindrome

→End

Yes↓

Move left pointer right

↓

Move right pointer left

↓

Left pointer >= Right pointer?

No→Compare characters again

Yes↓

All matched: Palindrome

→End

We check characters from both ends moving inward. If all pairs match, it's a palindrome.

Execution Sample

DSA Rust

fn is_palindrome(s: &str) -> bool {
    let bytes = s.as_bytes();
    let mut left = 0;
    let mut right = bytes.len().saturating_sub(1);
    while left < right {
        if bytes[left] != bytes[right] { return false; }
        left += 1;
        right -= 1;
    }
    true
}

This code checks if a string is a palindrome by comparing characters from both ends.

Execution Table

Step	Operation	Left Pointer	Right Pointer	Characters Compared	Match?	Visual State
1	Initialize pointers	0	6	-	-	"r a c e c a r"
2	Compare characters	0	6	'r' vs 'r'	Yes	"r a c e c a r"
3	Move pointers	1	5	-	-	"r a c e c a r"
4	Compare characters	1	5	'a' vs 'a'	Yes	"r a c e c a r"
5	Move pointers	2	4	-	-	"r a c e c a r"
6	Compare characters	2	4	'c' vs 'c'	Yes	"r a c e c a r"
7	Move pointers	3	3	-	-	"r a c e c a r"
8	Check pointers	3	3	-	-	Left >= Right, stop
9	Result	-	-	-	-	Palindrome detected (true)

💡 Left pointer reached or passed right pointer with all matches, confirming palindrome.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	Final
left	0	1	2	3	3
right	6	5	4	3	3
match_result	undefined	true	true	true	true

Key Moments - 3 Insights

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

Because when left >= right, all character pairs have been checked. See execution_table row 8 where pointers meet, meaning the middle is reached and palindrome is confirmed.

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

The function immediately returns false, indicating the string is not a palindrome. This is shown in execution_table row 2 condition check; if mismatch occurs, we exit early.

Why do we use bytes instead of characters directly?

Using bytes allows direct index access for comparison. Rust strings are UTF-8, so indexing characters is complex. Here, ASCII or simple byte comparison works for palindrome detection.

Visual Quiz - 3 Questions

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

A 'c' vs 'c'

B 'a' vs 'a'

C 'e' vs 'e'

D 'r' vs 'r'

💡 Hint

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

At which step does the left pointer first move to index 2?

A Step 3

B Step 5

C Step 7

D Step 2

💡 Hint

Look at the 'Left Pointer' values in variable_tracker after each step.

If the characters at step 4 did not match, what would happen?

A The function would continue checking next pairs.

B The function would return false immediately.

C The pointers would move inward anyway.

D The function would return true immediately.

💡 Hint

Refer to the 'Match?' column and the explanation in key_moments about mismatch behavior.

Concept Snapshot

Palindrome Detection:
- Use two pointers: left at start, right at end
- Compare characters at pointers
- If mismatch, return false
- Move pointers inward
- Stop when left >= right
- If all match, string is palindrome

Full Transcript

Palindrome detection checks if a string reads the same forwards and backwards. We use two pointers starting at the string's ends. We compare characters at these pointers. If they match, we move pointers inward and repeat. If any pair does not match, we stop and return false. When pointers meet or cross, all pairs matched, so we return true. This method efficiently checks palindrome property by comparing pairs only once.

Practice

1. What does a palindrome string mean in simple terms?

easy

A. It has all unique characters

B. It contains only vowels

C. It is always an even length string

D. It reads the same forwards and backwards

Solution

1. Step 1: Understand palindrome definition

   A palindrome is a word or phrase that reads the same from left to right and right to left.

2. Step 2: Check options against definition

   Only It reads the same forwards and backwards matches this meaning exactly.

3. Final Answer:

   It reads the same forwards and backwards -> Option D

4. Quick Check:

   Palindrome = Same forwards and backwards [OK]

Hint: Palindrome means mirror reading same both ways [OK]

Common Mistakes:

• Confusing palindrome with unique characters
• Thinking palindrome must be even length
• Assuming palindrome only has vowels

2. Which Rust code snippet correctly checks if a string s is a palindrome?

easy

A. s.reverse() == s

B. s.chars().eq(s.chars().rev())

C. s == s.reverse()

D. s.chars().all(|c| c == s.chars().next().unwrap())

Solution

1. Step 1: Understand Rust string palindrome check

   To check palindrome, compare characters iterator with its reverse iterator.

2. Step 2: Analyze options

   s.chars().eq(s.chars().rev()) uses chars().eq(chars().rev()) which correctly compares forward and backward characters. Options A and C use invalid methods or wrong comparisons. s.chars().all(|c| c == s.chars().next().unwrap()) checks if all characters are same, not palindrome.

3. Final Answer:

   s.chars().eq(s.chars().rev()) -> Option B

4. Quick Check:

   Compare chars iterator with reversed iterator [OK]

Hint: Use chars().eq(chars().rev()) to check palindrome [OK]

Common Mistakes:

• Using reverse() method on &str which does not exist
• Comparing string with reversed string incorrectly
• Checking all chars equal instead of palindrome

3. What is the output of this Rust code?

let s = "racecar";
let result = s.chars().eq(s.chars().rev());
println!("{}", result);

medium

A. true

B. Runtime panic

C. Compilation error

D. false

Solution

1. Step 1: Analyze the string and palindrome check

   The string "racecar" reads the same forwards and backwards, so it is a palindrome.

2. Step 2: Understand the code output

   The code compares characters forward and backward and returns true for palindrome strings. So, it prints true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   "racecar" palindrome check = true [OK]

Hint: Check if string equals its reverse chars [OK]

Common Mistakes:

• Assuming palindrome check returns false for odd length
• Thinking code causes compile or runtime error
• Confusing output with string itself

4. Identify the error in this Rust palindrome check code:

fn is_palindrome(s: &str) -> bool {
    s == s.chars().rev().collect::<String>()
}

medium

A. Cannot compare &str with String directly

B. Missing semicolon after collect

C. chars().rev() does not exist

D. Function return type should be String

Solution

1. Step 1: Understand types in comparison

   The left side is &str, right side is String from collect(). Direct comparison between &str and String is not allowed.

2. Step 2: Fix type mismatch

   Convert left side to String or right side to &str for comparison. For example, use s.to_string() == s.chars().rev().collect::<String>().

3. Final Answer:

   Cannot compare &str with String directly -> Option A

4. Quick Check:

   Type mismatch in comparison causes error [OK]

Hint: Compare same types: &str vs &str or String vs String [OK]

Common Mistakes:

• Ignoring type mismatch error
• Assuming collect needs semicolon inside expression
• Thinking chars().rev() is invalid method

5. You want to detect palindromes ignoring case and spaces in Rust. Which code snippet correctly implements this?

hard

A. let is_pal = s.chars().all(|c| c.is_alphanumeric());

B. let lower = s.to_lowercase();
let is_pal = lower == lower.reverse();

C. let filtered: String = s.chars().filter(|c| !c.is_whitespace()).collect();
let lower = filtered.to_lowercase();
let is_pal = lower.chars().eq(lower.chars().rev());

D. let filtered: String = s.chars().filter(|c| c.is_whitespace()).collect();
let is_pal = filtered == filtered.chars().rev().collect::<String>();

Solution

1. Step 1: Remove spaces and convert to lowercase

   let filtered: String = s.chars().filter(|c| !c.is_whitespace()).collect(); let lower = filtered.to_lowercase(); let is_pal = lower.chars().eq(lower.chars().rev()); filters out whitespace, collects characters, then converts to lowercase string.

2. Step 2: Check palindrome on cleaned string

   It compares characters forward and backward correctly on the cleaned lowercase string.

3. Final Answer:

   let filtered: String = s.chars().filter(|c| !c.is_whitespace()).collect(); let lower = filtered.to_lowercase(); let is_pal = lower.chars().eq(lower.chars().rev()); -> Option C

4. Quick Check:

   Filter spaces + lowercase + compare reversed chars [OK]

Hint: Clean string first: remove spaces, lowercase, then check palindrome [OK]

Common Mistakes:

• Using reverse() method which does not exist
• Filtering only whitespace but not converting case
• Checking all chars alphanumeric instead of palindrome