1608. The number-palindrome

Check if the given number is a palindrome.

A number is called a palindrome if it reads the same forwards and backwards.

Input. One single non-negative 32-bit integer.

Output. Print “Yes” if the number is a palindrome, and “No” otherwise.

Sample input	Sample output
121	Yes

SOLUTION

char array

Algorithm analysis

Read the input number as a string into the character array s. Let len be the length of the string s minus 1 (the index of the last character in the array). The string is a palindrome if for all 0 ≤ i ≤ len / 2, the condition s[i] = s[len − i] holds true.

For example, consider the number 4570754 with length 7. Then, len = 7 – 1 = 6. The following equalities hold:

· s[0] = s[6 – 0] = s[6] = ‘4’;

· s[1] = s[6 – 1] = s[5] = ‘5’;

· s[2] = s[6 – 2] = s[4] = ‘7’;

Algorithm implementation

Store the input number in the character array s.

char s[50];

Read the input number as a string. Set len equal to the index of the last digit.

gets(s);

len = strlen(s) - 1;

Initialize flag = 0, which means the number is a palindrome.

flag = 0;

If for some i the equality s[i] = s[len – i] is violated, the number is not a palindrome, and we set flag = 1.

for (i = 0; i <= len / 2; i++)

if (s[i] != s[len - i]) flag = 1;

Based on the value of the variable flag, print the result.

if (flag == 0) printf("Yes\n"); else printf("No\n");

Algorithm implementation – function IsPalindrome

Store the input number in the character array s.

char s[50];

The function IsPalindrome checks if the string s is a palindrome.

int IsPalindrome(char *s)

{

int i = 0, j = strlen(s) - 1;

while(i < j)

{

if (s[i] != s[j]) return 0;

i++; j--;

}

return 1;

}

The main part of the program. Read the input number as a string.

gets(s);

Check if the string s is a palindrome.

flag = IsPalindrome(s);

Based on the value of the variable flag, print the result.

if (flag == 1) printf("Yes\n"); else printf("No\n");

Algorithm implementation – STL reverse

Read the input number as a string p.

cin >> p;

Reverse the string p: q = reverse(p).

q = p; reverse(q.begin(),q.end());

If the strings p and q are identical, then the input string is a palindrome.

if (p == q) printf("Yes\n"); else printf("No\n");

Java implementation

import java.util.*;

public class Main

{

public static void main(String[] args)

{

Scanner con = new Scanner(System.in);

char[] s = con.nextLine().toCharArray();

int flag = 0, len = s.length - 1;

for(int i = 0; i < len - i; i++)

if (s[i] != s[len - i]) flag = 1;

if (flag == 0)

System.out.println("Yes");

else

System.out.println("No");

con.close();

}

}

Java implementation – fuction IsPalindrome

import java.util.*;

public class Main

{

public static int IsPalindrome(String s)

{

int i = 0, j = s.length() - 1;

while(i < j)

{

if (s.charAt(i) != s.charAt(j)) return 0;

i++; j--;

}

return 1;

}

public static void main(String[] args)

{

Scanner con = new Scanner(System.in);

String s = con.next();

int flag = IsPalindrome(s);

if (flag == 1)

System.out.println("Yes");

else

System.out.println("No");

con.close();

}

}

Python implementation

Read the input number as a string.

a = input()

The reverse of the string a can be obtained using slicing: a[::-1]. Check if the string a is equal to its reverse. Based on the result, print the corresponding answer.

if a == a[::-1]:

print("Yes")

else:

print("No")