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 are C and Python programs that implements above algorithm.

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 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;
}

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


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 and Python programs to implement reverse without using stack.

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 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;
}

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


Output:
Reversed string is cba

 

 

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