stack top() in C++ STL

Stacks are a type of container adaptors with LIFO(Last In First Out) type of work, where a new element is added at one end called the top of the stack, and an element is removed from the same end only.

stack::top() top() function is used to reference the top(or the newest) element of the stack.

Syntax :

stackname.top()

Parameters: No value is needed to pass as the parameter.
Return Value: Direct reference to the top element of the stack container.

Examples:

Input  : stackname.push(5);
         stackname.push(1);
         stackname.top();
Output : 1

Input  : stackname.push(5);
         stackname.push(1);
         stackname.push(2);
         stackname.top();
Output : 2

Errors and Exceptions

1. If the stack container is empty, it causes undefined behavior
2. It has a no exception throw guarantee if the stack is not empty

C++

// CPP program to illustrate
// Implementation of top() function
#include <iostream>
#include <stack>
using namespace std;

int main()
{
    stack<int> mystack;
    mystack.push(5);
    mystack.push(1);
    mystack.push(2);

    // Stack top
    cout << mystack.top();
    return 0;
}

Output:

2

Time Complexity: O(1)

Auxiliary Space: O(n)

Application :
Given a stack of integers, find the sum of the all the integers.  

Input : 1, 8, 3, 6, 2
Output: 20

Algorithm

1. Check if the stack is empty, if not add the top element to a variable initialized as 0, and pop the top element.
2. Repeat this step until the stack is empty.
3. Print the final value of the variable.

C++

// CPP program to illustrate
// Application of top() function
#include <iostream>
#include <stack>
using namespace std;

int main()
{
    int sum = 0;
    stack<int> mystack;
    mystack.push(1);
    mystack.push(8);
    mystack.push(3);
    mystack.push(6);
    mystack.push(2);

    // Stack becomes 1, 8, 3, 6, 2

    while (!mystack.empty()) {
        sum = sum + mystack.top();
        mystack.pop();
    }
    cout << sum;
    return 0;
}

Output:

20 

Time Complexity: O(n)

Auxiliary Space: O(n)