stack swap() 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 and (top) an element is removed from that end only.

This function is used to swap the contents of one stack with another stack of same type but the size may vary. Syntax :

stackname1.swap(stackname2)

Parameters: The name of the stack with which the contents have to be swapped. Result: All the elements of the 2 stack are swapped. Examples:

contents of the stack from top to bottom are
Input  : mystack1 = {4, 3, 2, 1}
         mystack2 = {9, 7 ,5, 3}
         mystack1.swap(mystack2);
Output : mystack1 =  9, 7, 5, 3
         mystack2 =  4, 3, 2, 1

Input  : mystack1 = {7, 5, 3, 1}
         mystack2 = {8, 6, 4, 2}
         mystack1.swap(mystack2);
Output : mystack1 =  8, 6, 4, 2
         mystack2 =  7, 5, 3, 1

Note: In stack container, the elements are printed in reverse order because the top is printed first then moving on to other elements. 

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

int main() 
{ 
        // stack container declaration 
    stack<int> mystack1; 
    stack<int> mystack2; 
    
    // pushing elements into first stack 
    mystack1.push(1); 
    mystack1.push(2); 
    mystack1.push(3); 
    mystack1.push(4); 
    
    // pushing elements into 2nd stack 
    mystack2.push(3); 
    mystack2.push(5); 
    mystack2.push(7); 
    mystack2.push(9); 

        // using swap() function to swap elements of stacks 
    mystack1.swap(mystack2); 

        // printing the first stack 
    cout<<"mystack1 = "; 
    while (!mystack1.empty()) { 
        cout<<mystack1.top()<<" "; 
        mystack1.pop(); 
    } 

        // printing the second stack 
    cout<<endl<<"mystack2 = "; 
    while (!mystack2.empty()) { 
        cout<<mystack2.top()<<" "; 
        mystack2.pop(); 
    } 
    return 0; 
} 

Output:

mystack1 = 9 7 5 3
mystack2 = 4 3 2 1

Time Complexity: O(1)

Auxiliary Space: O(n)