String Reversal Approaches in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse a string using the built-in reverse function.

DSA C++

#include <algorithm>
#include <string>
#include <iostream>

int main() {
    std::string s = "hello";
    std::reverse(s.[1](), s.end());
    std::cout << s << std::endl;
    return 0;
}

Drag options to blanks, or click blank then click option'

Abegin

Bstart

Cfront

Dfirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start()' or 'front()' which are not valid iterator methods.

Using 'first()' which does not exist for std::string.

The std::reverse function requires iterators to the beginning and end of the string. The correct method to get the start iterator is begin().

2fill in blank

medium

Complete the code to reverse a string by swapping characters using a loop.

DSA C++

#include <algorithm>
#include <string>
#include <iostream>

int main() {
    std::string s = "world";
    int n = s.size();
    for (int i = 0; i < n / 2; ++i) {
        std::swap(s[i], s[[1]]);
    }
    std::cout << s << std::endl;
    return 0;
}

Drag options to blanks, or click blank then click option'

Ai + 1

Bn - i - 1

Cn + i

Dn - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'n - 1' which always points to the last character only.

Using 'i + 1' which moves forward instead of backward.

To swap characters from start and end, the index from the end is n - i - 1.

3fill in blank

hard

Fix the error in the code that attempts to reverse a string using a while loop.

DSA C++

#include <string>
#include <iostream>

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

Drag options to blanks, or click blank then click option'

Aright - 1

Bleft

Cleft + 1

Dright

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning s[left] to itself causing no change.

Using left + 1 or right - 1 which skips characters.

To swap characters correctly, s[left] should be assigned s[right].

4fill in blank

hard

Fill both blanks to create a reversed copy of a string using a for loop.

DSA C++

#include <string>
#include <iostream>

int main() {
    std::string s = "reverse";
    std::string rev = "";
    for (int i = s.size() - 1; i >= 0; [1]) {
        rev.[2](s[i]);
    }
    std::cout << rev << std::endl;
    return 0;
}

Drag options to blanks, or click blank then click option'

A--i

B++i

Cpush_back

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using ++i which causes an infinite loop.

Using append which expects a string, not a single char.

The loop index should decrease with --i to move backward. The method push_back adds a character to the end of the string.

5fill in blank

hard

Fill the blanks to create a reversed string using a range-based for loop and prepending characters.

DSA C++

#include <string>
#include <iostream>

int main() {
    std::string s = "loop";
    std::string rev = "";
    for (char c : s) {
        rev = [1] + [2];
    }
    std::cout << rev << std::endl;
    return 0;
}

Drag options to blanks, or click blank then click option'

Arev

Bc

Crev + c

Dc + rev

Attempts:

3 left

💡 Hint

Common Mistakes

Appending instead of prepending causing no reversal.

Swapping the order incorrectly.

To reverse by prepending, assign rev = c + rev. This adds the current character before the existing string.

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