Learn String Reversal and Palindrome — Free DSA Tutorial

String Reversal

Reversing a string means producing a new string with the same characters in the opposite order, from last character to first, which can be done by building a new string from the end backward or by swapping characters in place.

Example: String Reversal

#include <iostream>
#include <algorithm>
using namespace std;
int main() {
    string s = "hello";
    reverse(s.begin(), s.end());
    cout << "Reversed: " << s << endl;
    return 0;
}

public class Main {
    public static void main(String[] args) {
        String s = "hello";
        String reversed = new StringBuilder(s).reverse().toString();
        System.out.println("Reversed: " + reversed);
    }
}

s = "hello"
print("Reversed:", s[::-1])

#include <stdio.h>
#include <string.h>
int main() {
    char s[] = "hello";
    int n = strlen(s);
    for (int i = 0; i < n / 2; i++) {
        char t = s[i]; s[i] = s[n - 1 - i]; s[n - 1 - i] = t;
    }
    printf("Reversed: %s\n", s);
    return 0;
}

Palindrome Basics

A palindrome is a string that reads identically forwards and backwards, like level or racecar. Checking for one means comparing the string against its own reverse, or comparing characters from both ends inward.

Example: Palindrome Basics

#include <iostream>
#include <algorithm>
using namespace std;
int main() {
    string s = "racecar";
    string rev = s;
    reverse(rev.begin(), rev.end());
    cout << (s == rev ? "Palindrome" : "Not a palindrome") << endl;
    return 0;
}

public class Main {
    public static void main(String[] args) {
        String s = "racecar";
        String rev = new StringBuilder(s).reverse().toString();
        System.out.println(s.equals(rev) ? "Palindrome" : "Not a palindrome");
    }
}

s = "racecar"
print("Palindrome" if s == s[::-1] else "Not a palindrome")

#include <stdio.h>
#include <string.h>
int main() {
    char s[] = "racecar";
    int n = strlen(s), isPal = 1;
    for (int i = 0; i < n / 2; i++) if (s[i] != s[n - 1 - i]) isPal = 0;
    printf("%s\n", isPal ? "Palindrome" : "Not a palindrome");
    return 0;
}

Two Pointer Method

The two-pointer method checks a palindrome without ever building a reversed copy: one pointer starts at the front, one at the back, and they compare characters while moving toward the middle, stopping early the moment a mismatch is found.

Example: Two Pointer Method

#include <iostream>
using namespace std;
int main() {
    string s = "level";
    int left = 0, right = s.length() - 1;
    bool isPal = true;
    while (left < right) {
        if (s[left] != s[right]) { isPal = false; break; }
        left++; right--;
    }
    cout << (isPal ? "Palindrome" : "Not a palindrome") << endl;
    return 0;
}

public class Main {
    public static void main(String[] args) {
        String s = "level";
        int left = 0, right = s.length() - 1;
        boolean isPal = true;
        while (left < right) {
            if (s.charAt(left) != s.charAt(right)) { isPal = false; break; }
            left++; right--;
        }
        System.out.println(isPal ? "Palindrome" : "Not a palindrome");
    }
}

s = "level"
left, right = 0, len(s) - 1
is_pal = True
while left < right:
    if s[left] != s[right]:
        is_pal = False
        break
    left += 1; right -= 1
print("Palindrome" if is_pal else "Not a palindrome")

#include <stdio.h>
#include <string.h>
int main() {
    char s[] = "level";
    int left = 0, right = strlen(s) - 1, isPal = 1;
    while (left < right) {
        if (s[left] != s[right]) { isPal = 0; break; }
        left++; right--;
    }
    printf("%s\n", isPal ? "Palindrome" : "Not a palindrome");
    return 0;
}

Ignoring Case

To make a palindrome check case-insensitive, convert both characters being compared to the same case (typically lowercase) before comparing, so Level is still correctly recognized as a palindrome.

Example: Ignoring Case

#include <iostream>
#include <cctype>
using namespace std;
int main() {
    string s = "Level";
    int left = 0, right = s.length() - 1;
    bool isPal = true;
    while (left < right) {
        if (tolower(s[left]) != tolower(s[right])) { isPal = false; break; }
        left++; right--;
    }
    cout << (isPal ? "Palindrome (case-insensitive)" : "Not a palindrome") << endl;
    return 0;
}

public class Main {
    public static void main(String[] args) {
        String s = "Level";
        int left = 0, right = s.length() - 1;
        boolean isPal = true;
        while (left < right) {
            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) { isPal = false; break; }
            left++; right--;
        }
        System.out.println(isPal ? "Palindrome (case-insensitive)" : "Not a palindrome");
    }
}

s = "Level"
left, right = 0, len(s) - 1
is_pal = True
while left < right:
    if s[left].lower() != s[right].lower():
        is_pal = False
        break
    left += 1; right -= 1
print("Palindrome (case-insensitive)" if is_pal else "Not a palindrome")

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main() {
    char s[] = "Level";
    int left = 0, right = strlen(s) - 1, isPal = 1;
    while (left < right) {
        if (tolower(s[left]) != tolower(s[right])) { isPal = 0; break; }
        left++; right--;
    }
    printf("%s\n", isPal ? "Palindrome (case-insensitive)" : "Not a palindrome");
    return 0;
}

Palindrome Practice

Palindrome problems are a great way to practice careful index management, since off-by-one errors at the string's boundaries are the most common bug, especially when also skipping non-alphanumeric characters.

Example: Palindrome Practice

#include <iostream>
#include <cctype>
using namespace std;
int main() {
    string s = "A man a plan a canal Panama";
    int left = 0, right = s.length() - 1;
    bool isPal = true;
    while (left < right) {
        if (!isalnum(s[left])) { left++; continue; }
        if (!isalnum(s[right])) { right--; continue; }
        if (tolower(s[left]) != tolower(s[right])) { isPal = false; break; }
        left++; right--;
    }
    cout << (isPal ? "Palindrome" : "Not a palindrome") << endl;
    return 0;
}

public class Main {
    public static void main(String[] args) {
        String s = "A man a plan a canal Panama";
        int left = 0, right = s.length() - 1;
        boolean isPal = true;
        while (left < right) {
            if (!Character.isLetterOrDigit(s.charAt(left))) { left++; continue; }
            if (!Character.isLetterOrDigit(s.charAt(right))) { right--; continue; }
            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) { isPal = false; break; }
            left++; right--;
        }
        System.out.println(isPal ? "Palindrome" : "Not a palindrome");
    }
}

s = "A man a plan a canal Panama"
left, right = 0, len(s) - 1
is_pal = True
while left < right:
    if not s[left].isalnum():
        left += 1; continue
    if not s[right].isalnum():
        right -= 1; continue
    if s[left].lower() != s[right].lower():
        is_pal = False
        break
    left += 1; right -= 1
print("Palindrome" if is_pal else "Not a palindrome")

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main() {
    char s[] = "A man a plan a canal Panama";
    int left = 0, right = strlen(s) - 1, isPal = 1;
    while (left < right) {
        if (!isalnum(s[left])) { left++; continue; }
        if (!isalnum(s[right])) { right--; continue; }
        if (tolower(s[left]) != tolower(s[right])) { isPal = 0; break; }
        left++; right--;
    }
    printf("%s\n", isPal ? "Palindrome" : "Not a palindrome");
    return 0;
}

String Reversal and Palindrome

String Reversal

Palindrome Basics

Two Pointer Method

Ignoring Case

Palindrome Practice

Chapter Quiz — Complete all 6 topics to unlock

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