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Using a Stack to Reverse Numbers

Yeah, I heard many of you saying this and I know it’s no big deal to reverse a number and neither is it using stack to do so. I am writing this just to give you an example of how certain things in a program can be done using stacks. So, let’s move on…

As many of you already know, a stack is a Data Structure in which data can be added and retrieved both from only one end (same end). Data is stored linearly and the last data added is the first one to be retrieved, due to this fact it is also known as Last-In-First-Out data structure. For more info please read Data Structures: Introduction to Stacks.

Now, let’s talk about reversing a number, well reversing means to rearrange a number in the opposite order from one end to the other.

Suppose we have a number

12345

then its reverse will be

54321

Ok, now let’s have a look at the example program which does this:


  // Program in C++ to reverse
  // a number using a Stack

  // PUSH -> Adding data to the sat ck
  // POP -> Retrieving data from the stack

  #include<iostream.h>

  // stack class
  class stack
  {
    int arr[100];
    // 'top' will hold the
    // index number in the
    // array from which all
    // the pushing and popping
    // will be done
    int top;
  public:
    stack();
    void push(int);
    int pop();
  };


  // member functions
  // of the stack class
  stack::stack()
  {
    // initialize the top
    // position
    top=-1;
  }

  void stack::push(int num)
  {
    if(top==100)
    {
      cout<<"\nStack Full!\n";
      return;
    }

    top++;
    arr[top]=num;
  }

  int stack::pop()
  {
    if(top==-1)
    {
      return NULL;
    }

    return arr[top--];
  }
  // member function definition ends

  void main()
  {
    stack st;
    int num, rev_num=0;
    int i=1, tmp;

    cout<<"Enter Number: ";
    cin>>num;

    // this code will store
    // the individual digits
    // of the number in the
    // Stack
    while(num>0)
    {
      st.push(num%10);
      num/=10;
    }

    // code below will retrieve
    // digits from the stack
    // to create the reversed
    // number
    tmp=st.pop();
    while(tmp!=NULL)
    {
      rev_num+=(tmp*i);
      tmp=st.pop();
      i*=10;
    }

    cout<<"Reverse: "<<rev_num<<endl;
  }

The above code is pretty much straightforward and I leave it up to you to understand it!

P.S. If you experience any problems understanding it please first read the article Data Structures: Introduction to Stacks

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