Stack | Set 3 (Reverse a string using stack)

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++



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// 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 reverese 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

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C

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// 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 reverese 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;
}

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Java

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/* 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);
    }
}

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Python

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# 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

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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++

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// 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 reverese 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

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C

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// 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 reverese 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;
}

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Java

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// 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 reverese 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

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Python

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# 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

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C#

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// 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 reverese 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

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Output:

Reversed string is cba

 

 

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