Which description best matches the palindrome-checking algorithm?

• A. A function uses indices i from 0 to n/2 and compares s[i] with s[n - i - 1], returning false on mismatch, true otherwise.
• B. A function sorts the string and checks if it equals its reverse.
• C. A function checks only the first and last characters.
• D. A function converts the string to uppercase and checks equality with its reverse.

Answer: (A)

Checking whether a string is a palindrome comes down to comparing characters that are opposite each other in the string. The described method uses an index that starts at the beginning and pairs it with the character at the mirrored position from the end, specifically comparing characters from both ends moving toward the center. If any mirrored pair differs, you can conclude immediately that the string isn’t a palindrome. If every pair matches, the string reads the same forward and backward.

This works because palindrome-hood requires every character to match its counterpart on the opposite side, and you only need to check up to the middle since the second half mirrors the first. It handles both even and odd lengths naturally—the middle character in an odd-length string doesn’t need a partner.

The algorithm is efficient: it runs in linear time relative to the string length and uses constant extra space, since it only tracks a couple of indices. Other approaches either rearrange characters (like sorting) and compare to a reversed version, which tests something different and is less efficient, or only check the outermost pair, which can miss mismatches deeper inside. If you wanted case-insensitive results, you’d need to normalize case before the comparisons, but the core idea remains the symmetric, half-length check.