Stack Data Structure (Introduction and Program) - JohnHau/mis GitHub Wiki
How to understand a stack practically? There are many real-life examples of a stack. Consider the simple example of plates stacked over one another in a canteen. The plate which is at the top is the first one to be removed, i.e. the plate which has been placed at the bottommost position remains in the stack for the longest period of time. So, it can be simply seen to follow the LIFO/FILO order.
Time Complexities of operations on stack:
push(), pop(), isEmpty() and peek() all take O(1) time. We do not run any loop in any of these operations.
Applications of stack:
Balancing of symbols Infix to Postfix /Prefix conversion Redo-undo features at many places like editors, photoshop. Forward and backward feature in web browsers Used in many algorithms like Tower of Hanoi, tree traversals, stock span problem, histogram problem. Backtracking is one of the algorithm designing techniques. Some examples of backtracking are the Knight-Tour problem, N-Queen problem, find your way through a maze, and game-like chess or checkers in all these problems we dive into someway if that way is not efficient we come back to the previous state and go into some another path. To get back from a current state we need to store the previous state for that purpose we need a stack. In Graph Algorithms like Topological Sorting and Strongly Connected Components In Memory management, any modern computer uses a stack as the primary management for a running purpose. Each program that is running in a computer system has its own memory allocations String reversal is also another application of stack. Here one by one each character gets inserted into the stack. So the first character of the string is on the bottom of the stack and the last element of a string is on the top of the stack. After Performing the pop operations on the stack we get a string in reverse order. Implementation: There are two ways to implement a stack:
Using array Using linked list
Implementing Stack using Arrays
/* C++ program to implement basic stack operations */ #include <bits/stdc++.h>
using namespace std;
#define MAX 1000
class Stack { int top;
public: int a[MAX]; // Maximum size of Stack
Stack() { top = -1; }
bool push(int x);
int pop();
int peek();
bool isEmpty();
};
bool Stack::push(int x) { if (top >= (MAX - 1)) { cout << "Stack Overflow"; return false; } else { a[++top] = x; cout << x << " pushed into stack\n"; return true; } }
int Stack::pop() { if (top < 0) { cout << "Stack Underflow"; return 0; } else { int x = a[top--]; return x; } } int Stack::peek() { if (top < 0) { cout << "Stack is Empty"; return 0; } else { int x = a[top]; return x; } }
bool Stack::isEmpty() { return (top < 0); }
// Driver program to test above functions int main() { class Stack s; s.push(10); s.push(20); s.push(30); cout << s.pop() << " Popped from stack\n"; //print all elements in stack : cout<<"Elements present in stack : "; while(!s.isEmpty()) { // print top element in stack cout<<s.peek()<<" "; // remove top element in stack s.pop(); }
return 0;
} Output :
10 pushed into stack
20 pushed into stack
30 pushed into stack
30 Popped from stack
Top element is : 20
Elements present in stack : 20 10
Pros: Easy to implement. Memory is saved as pointers are not involved.
Cons: It is not dynamic. It doesn’t grow and shrink depending on needs at runtime.
Implementing Stack using Linked List:
// C++ program for linked list implementation of stack #include <bits/stdc++.h> using namespace std;
// A structure to represent a stack class StackNode { public: int data; StackNode* next; };
StackNode* newNode(int data) { StackNode* stackNode = new StackNode(); stackNode->data = data; stackNode->next = NULL; return stackNode; }
int isEmpty(StackNode* root) { return !root; }
void push(StackNode** root, int data) { StackNode* stackNode = newNode(data); stackNode->next = *root; *root = stackNode; cout << data << " pushed to stack\n"; }
int pop(StackNode** root) { if (isEmpty(root)) return INT_MIN; StackNode temp = *root; *root = (*root)->next; int popped = temp->data; free(temp);
return popped;
}
int peek(StackNode* root) { if (isEmpty(root)) return INT_MIN; return root->data; }
// Driver code int main() { StackNode* root = NULL;
push(&root, 10);
push(&root, 20);
push(&root, 30);
cout << pop(&root) << " popped from stack\n";
cout << "Top element is " << peek(root) << endl;
cout<<"Elements present in stack : ";
//print all elements in stack :
while(!isEmpty(root))
{
// print top element in stack
cout<<peek(root)<<" ";
// remove top element in stack
pop(&root);
}
return 0;
}
// This is code is contributed by rathbhupendra Output:
10 pushed to stack 20 pushed to stack 30 pushed to stack 30 popped from stack Top element is 20 Elements present in stack : 20 10 Pros: The linked list implementation of a stack can grow and shrink according to the needs at runtime. Cons: Requires extra memory due to involvement of pointers.
We will cover the implementation of applications of the stack in separate posts.
Stack Set -2 (Infix to Postfix)
Quiz: Stack Questions
References: http://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29#Problem_Description
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