Palindrome or not

A palindrome is a word derived from the Greek meaning "running back again".

A string is called a palindrome string if the reverse of that string is the same as the original string. Example lol, madam, level.

Eg: original string = mom

after reversing the original string = mom

original == reversed string then the string is palindrome.

1. Check palindrome string

For reversing the string you can refer to my blog (I used 3 different approaches for reversing the string with an explanation).

reverse of a string

public class Palindrome {
  public static void main(String[] args) {
    String str = "madam";
    String reverseStr="";
    String s = str.toLowerCase();
    System.out.println("Original:"+s);
    for(int i=s.length()-1;i>=0;i--){
      reverseStr += s.charAt(i);
    }
    System.out.println("Reversed:"+str);
    if(s. equals(reverseStr)){
        System.out.println(s + " is Palindrome.");
    }
    else{
        System.out.println(s + " is not Palindrome.");
    }
  }
}

Original:madam
Reversed:madam
madam is Palindrome.

check palindrome string without reversing the string

public class Palindrome {
  public static void main(String[] args) {
    String str = "madam";
    String s = str.toLowerCase();
    int start = 0, end = s.length()-1;

    while(start < end){
      if(s.charAt(start)!=s.charAt(end)){
        System.out.println(s + " is not Palindrome.");
        System.exit(0);
      }
      start++;
      end--;
    }
    System.out.println(s + " is Palindrome.");
  }
}

madam is Palindrome.

Take the example "Madam"

start =0, end = 5-1 = 4

0 1 2 3 4 <==>m a d a m

check the first and last index of the string

(m == m) => then increment the start and decrease the end

start++ => start = 1

end-- => end = 4-1=3

check until the condition is false(start< end)

if start!=end then the print string is not a palindrome and exits from the program.

2. Check the palindrome number

check palindrome number using string

Convert the number to a string.

String.valueOf(number) => It will convert the number to a string and then follow the same process.

public class Palindrome {
  public static void main(String[] args) {
    int number = 121;
    String string = String.valueOf(number);
    String s = string.toLowerCase();
    int start = 0, end = s.length()-1;

    while(start < end){
      if(s.charAt(start)!=s.charAt(end)){
        System.out.println(number + " is not Palindrome.");
        System.exit(0);
      }
      start++;
      end--;
    }
    System.out.println(number + " is Palindrome.");
  }
}

121 is Palindrome.

check palindrome number without converting to string

public class Palindrome {
  public static void main(String[] args) {
    int num = 353, reversedNum = 0, rem;
    int originalNum = num;

    while (num != 0) {
      rem = num % 10;
      reversedNum = reversedNum * 10 + rem;
      num /= 10;
    }

    if (originalNum == reversedNum) {
      System.out.println(originalNum + " is Palindrome.");
    }
    else {
      System.out.println(originalNum + " is not Palindrome.");
    }
  }
}

353 is Palindrome.

Take number = 353 and store it in originalNum because in the process the number value will change.

reversedNum=0, rem

while loop

num = 353 => rem = num%10 (It will give the reminder of the number)

rem = 3

reversedNum = reversedNum 10 + rem (0*10 +3 = 3)

reversedNum = 3

num = num/10 (num = 353/10 = 35)

num = 35 => rem = num%10 (It will give the reminder of the number)

rem = 5

reversedNum = reversedNum * 10 + rem (3*10 +5 = 35)

reversedNum = 35

num = num/10 (num = 35/10 = 3)

num = 3 => rem = num%10 (It will give the reminder of the number)

rem = 3

reversedNum = reversedNum * 10 + rem (35*10 +3 = 353)

reversedNum = 353

num = num/10 (num = 3/10 = 0)

while(num!=0) condition false

then check the originalNum and reversedNum then print the output.