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| Stacks |
Stacks
A stack is a First In Last Out (FILO) or Last In First Out (LIFO) linear data structure.
Imagine the way plates are stacked in a buffet restaurant. You can only pick plates from top otherwise the stack will collapse.
Some basics operations of stack are:
- Push() - Inserts the item on the top of the stack
- Pop() - Removes the top item (often times, it is a good idea to implement this function so that it returns the element it removed)
- isEmpty() - Check whether the stack is empty or not (returns a boolean)
- Size() - Return the number of items in the stack (All the operations can be done in O(1) time - depending on the implementation)
Implementation of a stack is possible using either arrays or linked lists. The following is a simple array implementation of the stack data structure with its most common operations.
//Stack implementation using array in C++
//You can also include<stack> and then use the C++ STL Library stack class.
#include <bits/stdc++.h>
using namespace std;
class Stack {
int t;
int arr[MaxN];
public:
Stack() {
t = 0;
}
int size() {
return t;
}
bool isEmpty() {
return t < 1;
}
int top() {
return arr[t];
}
void push(int x) {
if (++t >= MaxN) {
cout << "Stack is full" << '\n';
return;
}
arr[t] = x;
}
void pop() {
arr[t--] = 0;
}
};
int main() {
Stack st;
st.push(4);
st.push(3);
st.push(5);
while (!st.isEmpty()) {
cout << st.size() << ' ' << st.top() << '\n';
st.pop();
}
return 0;
}
Using Arrays as Stacks
In some programming languages an array has stack functionality, allowing the developer to perform push and pop operations without the need for a custom stack data structure.
For example, an array in JavaScript has push and pop methods allowing one to easily implement stack functionality in an application.
stack = [];
let i = 0;
while(i < 5)
stack.push(i++);
while(stack.length) {
stack.pop();
}
A List in Python can also perform stack functionality in an application. Instead of push, one can use the append method.
stack = []
for i in range(5):
stack.append(i)
while len(stack):
stack.pop()
Applications
- Turn recursion into loop.
- Redo-Undo features.
- Sudoku solver
- Depth First Search.
- Tree traversals
- Infix expression -> Prefix/Postfix expression
- Valid Parentheses