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How to Reverse a String using Stack

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  • Difficulty Level : Easy
  • Last Updated : 17 Aug, 2022
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Given a string, reverse it using stack. 

Example:

Input: str = “GeeksQuiz”
Output: ziuQskeeG

Input: str = “abc”
Output: cba

Approach:

The idea is to create an empty stack and push all the characters from the string into it. Then pop each character one by one from the stack and put them back into the input string starting from the 0’th index. As we all know, stacks work on the principle of first in, last out. After popping all the elements and placing them back to string, the formed string would be reversed.

Follow the steps given below to reverse a string using stack. 

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

Below is the implementation of the above approach:

C++




// C++ 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 <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.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);
    }
}

Python3




# 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

Javascript




<script>
 
// Javascript program to reverse
// String using Stack
 
// stack
class Stack
{
    size;
    top;
    a = [];
  
    // Function to check if stack is empty
    isEmpty()
    {
        return(this.top < 0);
    }
     
    constructor(n)
    {
        this.top = -1;
        this.size = n;
        this.a = new Array(this.size);
    }
  
    // Function to push element in Stack
    push(x)
    {
        if (this.top >= this.size)
        {
            document.write("Stack Overflow<br>");
            return false;
        }
        else
        {
            this.a[++this.top] = x;
            return true;
        }
    }
  
    // Function to pop element from stack
    pop()
    {
        if (this.top < 0)
        {
            document.write("Stack Underflow<br>");
            return 0;
        }
        else
        {
            let x = this.a[this.top--];
            return x;
        }
    }
}
 
// Function to reverse the string
function reverse(str)
{
     
    // Create a stack of capacity
    // equal to length of string
    let n = str.length;
    let obj = new Stack(n);
      
    // Push all characters of string
    // to stack
    let 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++)
    {
        let ch = obj.pop();
        str[i] = ch;
    }
}
 
// Driver Code
let s = "GeeksQuiz".split("");
 
// Call reverse method
reverse(s);
 
// Print the reversed string
document.write("Reversed string is " +
               s.join(""));
 
// This code is contributed by rag2127
 
</script>

Output

Reversed string is ziuQskeeG

Time Complexity: O(N)  
Auxiliary Space: O(N) for Stack.

For other methods, please refer reverse a string.


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