Which statement best describes the algorithm to check if a string is a palindrome and its time complexity?

Which statement best describes the algorithm to check if a string is a palindrome and its time complexity?

• A. Check if the string contains no repeating characters.
• B. Compare characters from both ends moving toward the center; time complexity O(n).
• C. Convert the string to uppercase and perform character comparisons.
• D. Check only the first and last characters.

Answer: (B)

To determine if a string is a palindrome, you compare characters from the two ends and move toward the center. If every mirrored pair matches, the string reads the same forward and backward; if any pair mismatches, it isn’t a palindrome. You typically maintain two pointers, one at the start and one at the end, and advance them toward the middle until they meet. This approach examines at most half of the characters, so the time complexity is O(n) for a string of length n, and it uses constant extra space.

Why the other statements don’t fit: requiring no repeating characters isn’t necessary for palindromes—the same-character symmetry is what matters, and many palindromes contain repeats. converting to uppercase is a possible preprocessing step for case-insensitive checks, but it isn’t part of the essential palindrome test itself. checking only the first and last characters misses many mismatches that can occur inside the string.