C++ Program to 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++

// 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 the 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 the top is equal
// to the last index
int isFull(Stack* stack)
{ return stack->top == stack->capacity - 1; }

// Stack is empty when the top is equal to -1
int isEmpty(Stack* stack)
{ return stack->top == -1; }

// Function to add an item to stack.
// It increases the 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 the 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;
}

Java

import java.util.Arrays;

// A class to represent a stack
class Stack {
    int top;
    int capacity;
    char[] array;

    // function to create a stack of
    // given capacity. It initializes
    // size of the stack as 0
    public static Stack createStack(int capacity) {
        Stack stack = new Stack();
        stack.capacity = capacity;
        stack.top = -1;
        stack.array = new char[stack.capacity];
        return stack;
    }

    // Stack is full when the top is equal
    // to the last index
    public static boolean isFull(Stack stack) {
        return stack.top == stack.capacity - 1;
    }

    // Stack is empty when the top is equal to -1
    public static boolean isEmpty(Stack stack) {
        return stack.top == -1;
    }

    // Function to add an item to stack.
    // It increases the top by 1
    public static void push(Stack stack, char item) {
        if (isFull(stack))
            return;
        stack.array[++stack.top] = item;
    }

    // Function to remove an item from
    // stack. It decreases the top by 1
    public static char pop(Stack stack) {
        if (isEmpty(stack))
            return '\0';
        return stack.array[stack.top--];
    }
}

// A stack-based function to reverse
// a string
class ReverseString {
    static void reverse(char str[]) {
        // Create a stack of capacity
        // equal to the length of the string
        int n = str.length;
        Stack stack = Stack.createStack(n);

        // Push all characters of the string
        // to the stack
        for (int i = 0; i < n; i++)
            Stack.push(stack, str[i]);

        // Pop all characters of the string and
        // put them back to str
        for (int i = 0; i < n; i++)
            str[i] = Stack.pop(stack);
    }

    // Driver code
    public static void main(String[] args) {
        char[] str = "GeeksQuiz".toCharArray();

        reverse(str);
        System.out.println("Reversed string is " + Arrays.toString(str));
    }
}

// Contributed by Gaurav_Arora

Output:

Reversed string is ziuQskeeG

Time Complexity: O(n) where n is number of characters in stack. 
Auxiliary Space: O(n) for stack.