CODEWITHSUNDEEP
javastackpalindrome
wandermonk
wandermonk

Reputation: 7356

Palindrome using stack

I wrote a palindrome using stacks in java. But, due to some reason while popping the stack the program is not working as expected. I have also tried using the top and iterate using this variable. Either ways it is not working.

import java.util.Stack;

class Palindromer {

Stack<Character> stack = null;
int top = -1;

public boolean isPalindrome(String s) {
    for (int i = 0; i < s.length(); i++) {
        stack = new Stack<Character>();
        stack.push(s.charAt(i));
        top++;
    }

    System.out.println("value of top is " + top);
    String returnString = "";

    while (!stack.isEmpty()) {
        returnString += stack.pop();
    }
    System.out.println(returnString);

    return returnString.equals(s);
}
}

public class PalindromeChecker {
public static void main(String[] args) {
    Palindromer palin = new Palindromer();
    if (palin.isPalindrome("BananaB")) {
        System.out.println("is palindrome");
    } else {
        System.out.println("not a palindrome");
    }
}
}

Upvotes: 1

Views: 1167

Answers (3)

Wolf
Wolf

Reputation: 10238

You should place the new Stack<Character>(); outside the loop.

The way you do it:

    for (int i = 0; i < s.length(); i++) {
        stack = new Stack<Character>();
        stack.push(s.charAt(i));
        top++;
    }

the stack variable is reassigned each loop and does contain only one character after the loop. Change it into

    stack = new Stack<Character>();
    for (int i = 0; i < s.length(); i++) {
        stack.push(s.charAt(i));
        top++;
    }

BTW: Better declare stack and top in the isPalindrome method. So you get rid of the error in top with further calls.

Upvotes: 1

M4ver1k
M4ver1k

Reputation: 1575

Use the below version of isPalindrome method, Stack object was getting instantiated in each iteration of loop and hence you were not getting the expected behavior. 

public boolean isPalindrome(String s) {
    stack = new Stack<Character>(); 
    for (int i = 0; i < s.length(); i++) {
        stack.push(s.charAt(i));
        top++;
    }

    System.out.println("value of top is " + top);
    String returnString = "";

    while (!stack.isEmpty()) {
        returnString += stack.pop();
    }
    System.out.println(returnString);

    return returnString.equals(s);
}

Upvotes: 0

sol4me
sol4me

Reputation: 15698

You should initialise the stack outside the loop i.e.

for (int i = 0; i < s.length(); i++) {
        stack = new Stack<Character>();
        stack.push(s.charAt(i));
        top++;
    }

should be

stack = new Stack<Character>();
for (int i = 0; i < s.length(); i++) {

        stack.push(s.charAt(i));
        top++;
    }

Upvotes: 0

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