Question

This programming problem deals with the Linked List (LLStack) implementation.

a. Create a simple user interface to allow a user to input an expression and store it as a string variable.

b. Write a function using the LLStack implementation to check if the input string is a well-formed parenthetical expression. Assume (), {}, and [] are all parenthetical characters. Recall that a parenthetical expression is well-formed if

i. All closing parentheses match the most recent, unmatched opening parentheses of the same type.

ii. All opening parentheses have matching closing parentheses of the same type by the end of the expression.

The function should return true if the expression is well-formed, and false otherwise.

Answer 1

#include <bits/stdc++.h>
using namespace std;

// linked list node

struct Node {
   char data;
   struct Node* link;
};

//linked list head
struct Node* top;

// function to add an element data in the stack
// insert at the beginning
void push(char data)
{
   // create new node temp and allocate memory
   struct Node* temp;
   temp = new Node();

   // check if stack (heap) is full. Then inserting an element would
   // lead to stack overflow
   if (!temp) {
       cout << "\nHeap Overflow";
       exit(1);
   }

   // initialize data into temp data field
   temp->data = data;

   // put top pointer reference into temp link
   temp->link = top;

   // make temp as top of Stack
   top = temp;
}

// Utility function to check if the stack is empty or not
int isEmpty()
{
   return top == NULL;
}

// Utility function to return top element in a stack
char peek()
{
   // check for empty stack
   if (!isEmpty())
       return top->data;
   else
       exit(1);
}

// Utility function to pop top
// element from the stack

void pop()
{
   struct Node* temp;

   // check for stack underflow
   if (top == NULL) {
       cout << "\nStack Underflow" << endl;
       exit(1);
   }
   else {
       // top assign into temp
       temp = top;

       // assign second node to top
       top = top->link;

       // destroy connection between first and second
       temp->link = NULL;

       // release memory of top node
       free(temp);
   }
}

// Function to print all the
// elements of the stack
void display()
{
   struct Node* temp;

   // check for stack underflow
   if (top == NULL) {
       cout << "\nStack Underflow";
       exit(1);
   }
   else {
       temp = top;
       while (temp != NULL) {

           // print node data
           cout << temp->data << " ";

           // assign temp link to temp
           temp = temp->link;
       }
   }
}

bool balancedParanthesis(string expr)
{
  
char x;
  
// Traversing the Expression
for (int i=0; i<expr.length(); i++)
{
if (expr[i]=='('||expr[i]=='['||expr[i]=='{')
{
// Push the element in the stack
push(expr[i]);
continue;
}
  
// IF current current character is not opening
// bracket, then it must be closing. So stack
// cannot be empty at this point.
if (isEmpty())
return false;
  
switch (expr[i])
{
case ')':
  
// Store the top element in a
x = peek();
pop();
if (x=='{' || x=='[')
return false;
break;
  
case '}':
  
// Store the top element in b
x = peek();
pop();
if (x=='(' || x=='[')
return false;
break;
  
case ']':
  
// Store the top element in c
x = peek();
pop();
if (x =='(' || x == '{')
return false;
break;
}
}
  
// Check Empty Stack
return (isEmpty());
}

// Driver Code
int main()
{
  
   string s;
   cout<<"enter the expression : ";
   cin>>s;
   if(balancedParanthesis(s))
       cout<<"true"<<endl;
   else
       cout<<"false"<<endl;

   return 0;

}

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