String Reversal Approaches in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of string reversal using two-pointer swap

What is the output of the following C++ code that reverses a string using two pointers?

DSA C++

std::string s = "hello";
int left = 0, right = s.size() - 1;
while (left < right) {
    char temp = s[left];
    s[left] = s[right];
    s[right] = temp;
    left++;
    right--;
}
std::cout << s << std::endl;

A olleh

B hello

C SyntaxError

D ohlle

💡 Hint

Think about swapping characters from the start and end moving towards the center.

The code swaps characters from the start and end moving inward, effectively reversing the string.

❓ Predict Output

intermediate

Output of string reversal using recursion

What is the output of this recursive function that reverses a string?

DSA C++

std::string reverseString(const std::string& s) {
    if (s.empty()) return "";
    return reverseString(s.substr(1)) + s[0];
}

int main() {
    std::string s = "abc";
    std::cout << reverseString(s) << std::endl;
    return 0;
}

A abc

B RuntimeError

C SyntaxError

D cba

💡 Hint

The function calls itself with the substring excluding the first character and appends the first character at the end.

The recursion builds the reversed string by moving the first character to the end after reversing the rest.

🔧 Debug

advanced

Identify the error in this string reversal code

What error does this code produce when trying to reverse a string?

DSA C++

std::string s = "test";
for (int i = 0; i <= s.size() / 2; i++) {
    char temp = s[i];
    s[i] = s[s.size() - i - 1];
    s[s.size() - i - 1] = temp;
}
std::cout << s << std::endl;

A Out of range error

B No error, outputs 'tset'

C Syntax error

D Infinite loop

💡 Hint

Check the index used for swapping characters carefully.

The code accesses s[s.size() - i], which is out of range because string indices go from 0 to size()-1.

🧠 Conceptual

advanced

Which approach uses O(n) extra space for string reversal?

Among these string reversal methods, which one uses extra space proportional to the string length?

A In-place two-pointer swap

B Using recursion with substring concatenation

C Using std::reverse from <algorithm>

D Swapping characters with a temporary variable

💡 Hint

Consider which method creates new strings during the process.

Recursion with substring concatenation creates new strings at each call, using O(n) extra space.

🚀 Application

expert

Final string state after multiple reversal operations

Given the following operations on string s, what is the final string?

DSA C++

std::string s = "algorithm";
// Operation 1: Reverse entire string
std::reverse(s.begin(), s.end());
// Operation 2: Reverse substring from index 2 to 5 (inclusive)
std::reverse(s.begin() + 2, s.begin() + 6);
// Operation 3: Reverse substring from index 0 to 3 (inclusive)
std::reverse(s.begin(), s.begin() + 4);
std::cout << s << std::endl;

A mhtirogla

B mhtorgila

C rohmitgla

D mhtroigla

💡 Hint

Apply each reversal step carefully on the string.

Step 1 reverses entire string to 'mhtirogla'. Step 2 reverses substring indices 2-5 ('t','i','r','o') to ('o','r','i','t') resulting in 'mhoritgla'. Step 3 reverses substring indices 0-3 ('m','h','o','r') to ('r','o','h','m') resulting in 'rohmitgla'.

Practice

1. What does the std::reverse function do when applied to a string in C++?

easy

A. It reverses the order of characters in the string.

B. It sorts the characters in ascending order.

C. It removes all vowels from the string.

D. It converts all characters to uppercase.

Solution

1. Step 1: Understand std::reverse behavior

   The std::reverse function reverses the order of elements in a range, here characters in a string.

2. Step 2: Apply to string

   When applied to a string, it changes the string so the last character becomes first, and so on.

3. Final Answer:

   It reverses the order of characters in the string. -> Option A

4. Quick Check:

   std::reverse reverses string characters [OK]

Hint: std::reverse flips string characters in place [OK]

Common Mistakes:

• Thinking std::reverse sorts characters
• Confusing reversal with uppercase conversion
• Assuming it removes characters

2. Which of the following is the correct syntax to reverse a string s using std::reverse in C++?

easy

A. std::reverse(s.begin(), s.end());

B. std::reverse(s, 0, s.length());

C. reverse(s.begin(), s.end());

D. std::reverse(s.start(), s.finish());

Solution

1. Step 1: Identify correct std::reverse usage

   The std::reverse function requires two iterators: start and end of the range.

2. Step 2: Match syntax with string iterators

   For string s, s.begin() and s.end() provide correct iterators.

3. Final Answer:

   std::reverse(s.begin(), s.end()); -> Option A

4. Quick Check:

   std::reverse needs begin and end iterators [OK]

Hint: Use s.begin() and s.end() with std::reverse [OK]

Common Mistakes:

• Using incorrect iterator names like start() or finish()
• Calling reverse without std:: prefix
• Passing string and indices directly

3. What will be the output of the following C++ code?

std::string s = "hello";
for (int i = 0; i < s.length() / 2; ++i) {
    std::swap(s[i], s[s.length() - 1 - i]);
}
std::cout << s;

medium

A. hello

B. helol

C. olleh

D. error

Solution

1. Step 1: Understand the swapping loop

   The loop swaps characters from start and end moving towards the center, effectively reversing the string.

2. Step 2: Trace the swaps on "hello"

   Swap s[0] and s[4]: 'h' and 'o' -> "oellh"; Swap s[1] and s[3]: 'e' and 'l' -> "olleh".

3. Final Answer:

   olleh -> Option C

4. Quick Check:

   Swapping pairs reverses string [OK]

Hint: Swapping pairs from ends reverses string [OK]

Common Mistakes:

• Not swapping correctly causing partial reversal
• Loop running too many or too few times
• Confusing indices causing wrong output

4. Identify the error in the following code that attempts to reverse a string s.

for (int i = 0; i <= s.length() / 2; ++i) {
    std::swap(s[i], s[s.length() - i]);
}

medium

A. No error; code correctly reverses the string.

B. The second index should be s.length() - 1 - i instead of s.length() - i to avoid out-of-range access.

C. The loop should start from i = 1 instead of 0.

D. std::swap cannot be used with string characters.

Solution

1. Step 1: Check the swap indices

   The code uses s[s.length() - i]. For i=0, this accesses s[s.length()], out of bounds.

2. Step 2: Correct reversal index

   To reverse, swap s[i] with s.length() - 1 - i.

3. Final Answer:

   The second index should be s.length() - 1 - i instead of s.length() - i to avoid out-of-range access. -> Option B

4. Quick Check:

   Missing -1 in second index causes out-of-range [OK]

Hint: Check indices carefully to avoid out-of-range errors [OK]

Common Mistakes:

• Off-by-one error: s.length() - i instead of s.length() - 1 - i
• Incorrect index calculation for swap
• Assuming std::swap can't swap chars

5. Given a string s, which approach below correctly reverses the string without using extra space and handles empty strings safely?

hard

A. Use a while loop swapping characters until i > s.length().

B. Use std::reverse(s.begin(), s.end() - 1); to reverse all but last character.

C. Create a new string and append characters from end to start.

D. Use a for loop swapping characters from start to end with condition i < s.length() / 2.

Solution

1. Step 1: Analyze each approach

   Use a for loop swapping characters from start to end with condition i < s.length() / 2. swaps characters in place up to half length, safe for empty strings. Use std::reverse(s.begin(), s.end() - 1); to reverse all but last character. misses last character. Create a new string and append characters from end to start. uses extra space. Use a while loop swapping characters until i > s.length(). has wrong loop condition.

2. Step 2: Confirm correctness and safety

   Use a for loop swapping characters from start to end with condition i < s.length() / 2. reverses string in place without extra space and works for empty strings as loop won't run.

3. Final Answer:

   Use a for loop swapping characters from start to end with condition i < s.length() / 2. -> Option D

4. Quick Check:

   In-place swap up to half length reverses string safely [OK]

Hint: Swap pairs up to half length for safe in-place reversal [OK]

Common Mistakes:

• Using incorrect end iterator causing partial reversal
• Creating new string uses extra space
• Wrong loop conditions causing infinite loops