C palindrome check example
Yes, checking whether the string is palindrome can be achieved through the double-pointer method; 1. Use left and right two pointers to move from the beginning and end of the string to the center respectively; 2. In the basic version, directly compare whether the characters are equal, case sensitive and not ignore spaces and punctuations; 3. In the enhanced version, non-alphanumeric characters are skipped and converted to lowercase for comparison, so as to realize the function of ignoring case, spaces and punctuations; 4. If all corresponding characters match, the string is palindrome, otherwise it is not; the time complexity of this method is O(n) and the space complexity is O(1), which is suitable for handling various boundary situations such as empty strings, single characters and mixed-format text.
Checking if a string is a palindrome in C is a common programming task. A palindrome is a word, phrase, or sequence that reads the same forwards and backwards (ignoring case, spaces, and punctuation in some cases).
Here's a simple and clear C example that checks whether a given string is a palindrome:
Basic Palindrome Check (Case-Sensitive, Ignores Spaces/Punctuation)
#include <iostream>
#include <string>
#include <algorithm>
bool isPalindrome(const std::string& str) {
int left = 0;
int right = str.length() - 1;
while (left < right) {
if (str[left] != str[right]) {
return false;
}
left ;
right--;
}
return true;
}
int main() {
std::string input;
std::cout << "Enter a string: ";
std::getline(std::cin, input);
if (isPalindrome(input)) {
std::cout << "\"" << input << "\" is a palindrome.\n";
} else {
std::cout << "\"" << input << "\" is not a palindrome.\n";
}
return 0;
}Enhanced Version (Ignores Case, Spaces, and Punctuation)
If you want to check palindromes more realistically (like "A man a plan a canal Panama"), use this version:
#include <iostream>
#include <string>
#include <cctype> // for tower, isalnum
bool isPalindromeEnhanced(const std::string& str) {
int left = 0;
int right = str.length() - 1;
while (left < right) {
// Skip non-alphanumeric characters from the left
while (left < right && !std::isalnum(str[left])) {
left ;
}
// Skip non-alphanumeric characters from the right
while (left < right && !std::isalnum(str[right])) {
right--;
}
// Compare characters (case-insensitive)
if (std::tolower(str[left]) != std::tolower(str[right])) {
return false;
}
left ;
right--;
}
return true;
}
int main() {
std::string input;
std::cout << "Enter a string: ";
std::getline(std::cin, input);
if (isPalindromeEnhanced(input)) {
std::cout << "\"" << input << "\" is a palindrome.\n";
} else {
std::cout << "\"" << input << "\" is not a palindrome.\n";
}
return 0;
}Example Outputs
- Input:
racecar→ Output: is a palindrome - Input:
hello→ Output: not a palindrome - Input:
A man a plan a canal Panama→ Output: is a palindrome (with enhanced version)
Key Points
- The two-pointer approach (left and right) is efficient: O(n) time, O(1) space.
- Use
std::isalnum()andstd::tolower()to handle real-world text. - Always consider edge cases: empty strings, single characters, mixed case, punctuation.
Basically, just compare characters from both ends moving inward — if all match, it's a palindrome.
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