Stack Example 1: Reverse a string or word
In our previous article Stack Introduction and Implementation we had learnt how to implement stack in java. In this first example we will see how to reverse a word or a string using a stack.
When you run the following code, it will ask you to type a string. Once you type the string, press Enter, then it will print the string with letters in reverse order.
Stack Example 1: Reverse a string
package com.sneppets.dsalgo;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
class MyStackImpl
{
//max stack array size
private int maxStackArraySize;
//stack array
private char[] myStackArray;
//stack top
private int top;
//constructor
public MyStackImpl(int size)
{
//set stack array size
maxStackArraySize = size;
//create array
myStackArray = new char[maxStackArraySize];
//no elements in the stack array yet
top = -1;
}
public void push(char element)
{
//increment top and insert the element in top of the stack array
myStackArray[++top] = element;
}
public char pop()
{
//access the element from the top of the stack array
return myStackArray[top--];
}
public char peek()
{
//peek at the top of the stack array
return myStackArray[top];
}
public boolean isEmpty()
{
//return true is stack array is empty
return (top == -1);
}
public boolean isFull()
{
//return true if stack array is full
return (top == maxStackArraySize-1);
}
}
/**
*
* @author sneppets.com
*
*/
class StackReverseString
{
private String inputStr;
private String outputStr;
public StackReverseString(String inStr)
{
inputStr = inStr;
}
public String reverseInputString()
{
int maxStackSize = inputStr.length();
MyStackImpl myStack = new MyStackImpl(maxStackSize);
for (int i=0; i<inputStr.length(); i++)
{
char ch = inputStr.charAt(i);
myStack.push(ch);
}
outputStr = "";
while( !myStack.isEmpty())
{
char ch = (char) myStack.pop();
outputStr = outputStr + ch;
}
return outputStr;
}
}
public class StackReverseStringExample {
public static void main (String[] args) throws IOException
{
String inStr, outStr;
while(true)
{
System.out.println("Enter a String to reverse: ");
System.out.flush();
inStr = getInputString();
if(inStr.equals(""))
break;
StackReverseString stackReverseWord = new StackReverseString(inStr);
outStr = stackReverseWord.reverseInputString();
System.out.println("Reversed output string: " + outStr);
}
}
public static String getInputString() throws IOException {
InputStreamReader isr = new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(isr);
String inStr = br.readLine();
return inStr;
}
}
Output
Enter a String to reverse: Hello Reversed output string: olleH
Observation
A stack is used to reverse the letters in a string. Because stack follows Last In First Out (LIFO) principle, the stack reverses the order of characters.