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Stack | Set 3 (Reverse a string using stack)
  • Difficulty Level : Easy
  • Last Updated : 16 Apr, 2021

Given a string, reverse it using stack. For example “GeeksQuiz” should be converted to “ziuQskeeG”.
Following is simple algorithm to reverse a string using stack. 
 

1) Create an empty stack.
2) One by one push all characters of string to stack.
3) One by one pop all characters from stack and put 
   them back to string.

 

Following programs implements above algorithm. 
 

C++




// CPP program to reverse a string using stack
#include <bits/stdc++.h>
using namespace std;
 
// A structure to represent a stack
class Stack
{
    public:
    int top;
    unsigned capacity;
    char* array;
};
 
// function to create a stack of given
// capacity. It initializes size of stack as 0
Stack* createStack(unsigned capacity)
{
    Stack* stack = new Stack();
    stack->capacity = capacity;
    stack->top = -1;
    stack->array = new char[(stack->capacity * sizeof(char))];
    return stack;
}
 
// Stack is full when top is equal to the last index
int isFull(Stack* stack)
{ return stack->top == stack->capacity - 1; }
 
// Stack is empty when top is equal to -1
int isEmpty(Stack* stack)
{ return stack->top == -1; }
 
// Function to add an item to stack.
// It increases top by 1
void push(Stack* stack, char item)
{
    if (isFull(stack))
        return;
    stack->array[++stack->top] = item;
}
 
// Function to remove an item from stack.
// It decreases top by 1
char pop(Stack* stack)
{
    if (isEmpty(stack))
        return -1;
    return stack->array[stack->top--];
}
 
// A stack based function to reverse a string
void reverse(char str[])
{
    // Create a stack of capacity
    //equal to length of string
    int n = strlen(str);
    Stack* stack = createStack(n);
 
    // Push all characters of string to stack
    int i;
    for (i = 0; i < n; i++)
        push(stack, str[i]);
 
    // Pop all characters of string and
    // put them back to str
    for (i = 0; i < n; i++)
        str[i] = pop(stack);
}
 
// Driver code
int main()
{
    char str[] = "GeeksQuiz";
 
    reverse(str);
    cout << "Reversed string is " << str;
 
    return 0;
}
 
// This code is contributed by rathbhupendra

C




// C program to reverse a string using stack
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <limits.h>
 
// A structure to represent a stack
struct Stack
{
    int top;
    unsigned capacity;
    char* array;
};
 
// function to create a stack of given
// capacity. It initializes size of stack as 0
struct Stack* createStack(unsigned capacity)
{
    struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));
    stack->capacity = capacity;
    stack->top = -1;
    stack->array = (char*) malloc(stack->capacity * sizeof(char));
    return stack;
}
 
// Stack is full when top is equal to the last index
int isFull(struct Stack* stack)
{ return stack->top == stack->capacity - 1; }
 
// Stack is empty when top is equal to -1
int isEmpty(struct Stack* stack)
{ return stack->top == -1; }
 
// Function to add an item to stack.
// It increases top by 1
void push(struct Stack* stack, char item)
{
    if (isFull(stack))
        return;
    stack->array[++stack->top] = item;
}
 
// Function to remove an item from stack.
// It decreases top by 1
char pop(struct Stack* stack)
{
    if (isEmpty(stack))
        return INT_MIN;
    return stack->array[stack->top--];
}
 
// A stack based function to reverse a string
void reverse(char str[])
{
    // Create a stack of capacity
    //equal to length of string
    int n = strlen(str);
    struct Stack* stack = createStack(n);
 
    // Push all characters of string to stack
    int i;
    for (i = 0; i < n; i++)
        push(stack, str[i]);
 
    // Pop all characters of string and
    // put them back to str
    for (i = 0; i < n; i++)
        str[i] = pop(stack);
}
 
// Driver program to test above functions
int main()
{
    char str[] = "GeeksQuiz";
 
    reverse(str);
    printf("Reversed string is %s", str);
 
    return 0;
}

Java




/* Java program to reverse
String using Stack */
     
import java.util.*;
 
//stack
class Stack
{
    int size;
    int top;
    char[] a;
 
    //function to check if stack is empty
    boolean isEmpty()
    {
        return (top < 0);
    }
     
    Stack(int n)
    {
        top = -1;
        size = n;
        a = new char[size];
    }
 
    //function to push element in Stack
    boolean push(char x)
    {
        if (top >= size)
        {
        System.out.println("Stack Overflow");
        return false;
        }
        else
        {
            a[++top] = x;
            return true;
        }
    }
 
    //function to pop element from stack
    char pop()
    {
        if (top < 0)
        {
        System.out.println("Stack Underflow");
        return 0;
        }
        else
        {
            char x = a[top--];
            return x;
        }
    }
}
 
 
// Driver code
class Main
{
    //function to reverse the string
    public static void reverse(StringBuffer str)
    {
        // Create a stack of capacity
        // equal to length of string
        int n = str.length();
        Stack obj = new Stack(n);
         
        // Push all characters of string
        // to stack
        int i;
        for (i = 0; i < n; i++)
        obj.push(str.charAt(i));
     
        // Pop all characters of string
        // and put them back to str
        for (i = 0; i < n; i++)
        {
            char ch = obj.pop();
            str.setCharAt(i,ch);
        }
    }
     
    //driver function
    public static void main(String args[])
    {
        //create a new string
        StringBuffer s= new StringBuffer("GeeksQuiz");
         
        //call reverse method
        reverse(s);
         
        //print the reversed string
        System.out.println("Reversed string is " + s);
    }
}

Python




# Python program to reverse a string using stack
 
# Function to create an empty stack.
# It initializes size of stack as 0
def createStack():
    stack=[]
    return stack
 
# Function to determine the size of the stack
def size(stack):
    return len(stack)
 
# Stack is empty if the size is 0
def isEmpty(stack):
    if size(stack) == 0:
        return true
 
# Function to add an item to stack .
# It increases size by 1
def push(stack,item):
    stack.append(item)
 
#Function to remove an item from stack.
# It decreases size by 1
def pop(stack):
    if isEmpty(stack): return
    return stack.pop()
 
# A stack based function to reverse a string
def reverse(string):
    n = len(string)
     
    # Create a empty stack
    stack = createStack()
 
    # Push all characters of string to stack
    for i in range(0,n,1):
        push(stack,string[i])
 
    # Making the string empty since all
    #characters are saved in stack
    string=""
 
    # Pop all characters of string and
    # put them back to string
    for i in range(0,n,1):
        string+=pop(stack)
         
    return string
     
# Driver program to test above functions
string="GeeksQuiz"
string = reverse(string)
print("Reversed string is " + string)
 
# This code is contributed by Sunny Karira

C#




/* C# program to reverse
String using Stack */
using System;
using System.Text;
//stack
class Stack
{
    public int size;
    public int top;
    public char[] a;
 
    // function to check if stack is empty
    public Boolean isEmpty()
    {
        return (top < 0);
    }
     
    public Stack(int n)
    {
        top = -1;
        size = n;
        a = new char[size];
    }
 
    // function to push element in Stack
    public Boolean push(char x)
    {
        if (top >= size)
        {
            Console.WriteLine("Stack Overflow");
            return false;
        }
        else
        {
            a[++top] = x;
            return true;
        }
    }
 
    // function to pop element from stack
    public char pop()
    {
        if (top < 0)
        {
            Console.WriteLine("Stack Underflow");
            return ' ';
        }
        else
        {
            char x = a[top--];
            return x;
        }
    }
}
 
class GFG
{
    // function to reverse the string
    public static void reverse(StringBuilder str)
    {
        // Create a stack of capacity
        // equal to length of string
        int n = str.Length;
        Stack obj = new Stack(n);
         
        // Push all characters of string
        // to stack
        int i;
        for (i = 0; i < n; i++)
        obj.push(str[i]);
     
        // Pop all characters of string
        // and put them back to str
        for (i = 0; i < n; i++)
        {
            char ch = obj.pop();
            str[i] = ch;
        }
    }
     
    // Driver code
    public static void Main(String []args)
    {
        // create a new string
        StringBuilder s = new StringBuilder("GeeksQuiz");
         
        // call reverse method
        reverse(s);
         
        // print the reversed string
        Console.WriteLine("Reversed string is " + s);
    }
}
 
// This code is contributed by Rajput-Ji

Output: 

Reversed string is ziuQskeeG

Time Complexity: O(n) where n is number of characters in stack. 
Auxiliary Space: O(n) for stack.
A string can also be reversed without using any auxiliary space. Following C, Java, C# and Python programs to implement reverse without using stack. 
 



C++




// C++ program to reverse a string without using stack
#include <bits/stdc++.h>
using namespace std;
 
// A utility function to swap two characters
void swap(char *a, char *b)
{
    char temp = *a;
    *a = *b;
    *b = temp;
}
 
// A stack based function to reverse a string
void reverse(char str[])
{
    // get size of string
    int n = strlen(str), i;
 
    for (i = 0; i < n/2; i++)
        swap(&str[i], &str[n-i-1]);
}
 
// Driver program to test above functions
int main()
{
    char str[] = "abc";
 
    reverse(str);
    cout<<"Reversed string is "<< str;
 
    return 0;
}
 
//This is code is contributed by rathbhupendra

C




// C program to reverse a string without using stack
#include <stdio.h>
#include <string.h>
 
// A utility function to swap two characters
void swap(char *a, char *b)
{
    char temp = *a;
    *a = *b;
    *b = temp;
}
 
// A stack based function to reverse a string
void reverse(char str[])
{
    // get size of string
    int n = strlen(str), i;
 
    for (i = 0; i < n/2; i++)
        swap(&str[i], &str[n-i-1]);
}
 
// Driver program to test above functions
int main()
{
    char str[] = "abc";
 
    reverse(str);
    printf("Reversed string is %s", str);
 
    return 0;
}

Java




// Java program to reverse a string without using stack
public class GFG {
// A utility function to swap two characters
 
    static void swap(char a[], int index1, int index2) {
        char temp = a[index1];
        a[index1] = a[index2];
        a[index2] = temp;
    }
 
// A stack based function to reverse a string
    static void reverse(char str[]) {
        // get size of string
        int n = str.length, i;
 
        for (i = 0; i < n / 2; i++) {
            swap(str, i, n - i - 1);
        }
    }
 
// Driver program to test above functions
    public static void main(String[] args) {
        char str[] = "abc".toCharArray();
 
        reverse(str);
        System.out.printf("Reversed string is " + String.valueOf(str));
    }
}
// This code is contributed by 29AjayKumar

Python




# Python program to reverse a string without stack
 
# Function to reverse a string
def reverse(string):
    string = string[::-1]
    return string
 
# Driver program to test above functions
string = "abc"
string = reverse(string)
print("Reversed string is " + string)
 
# This code is contributed by Sunny Karira

C#




// C# program to reverse a string without using stack
using System;
public class GFG {
// A utility function to swap two characters
 
    static void swap(char []a, int index1, int index2) {
        char temp = a[index1];
        a[index1] = a[index2];
        a[index2] = temp;
    }
 
// A stack based function to reverse a string
    static void reverse(char []str) {
        // get size of string
        int n = str.Length, i;
 
        for (i = 0; i < n / 2; i++) {
            swap(str, i, n - i - 1);
        }
    }
 
// Driver program to test above functions
    public static void Main() {
        char []str = "abc".ToCharArray();
 
        reverse(str);
        Console.WriteLine("Reversed string is " + String.Join("",str));
    }
}
// This code is contributed by PrinciRaj1992

Javascript




<script>
 
// JavaScript program to reverse a string without using stack
 
// A stack based function to reverse a string
function reverse(str)
{
    // get size of string
    let n = (str).length;
    let i;
    let arr = str.split('');
    for (i = 0; i < n/2; i++)
    {
        let temp = arr[i];
        arr[i] = arr[n-i-1];
        arr[n-i-1] = temp;
    }
    return arr.join('');
}
 
// Driver program to test above functions
let str = "abc";
str = reverse(str);
document.write("Reversed string is " +str);
 
</script>

Output: 

Reversed string is cba

  
 

 
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 

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