# How to Reverse a Stack using Recursion ?

[Question] Write a program to reverse a stack using recursion.
Input: elements present in stack from top to bottom 1 2 3 4
Output: 4 3 2 1

``````// Stack Definition Starts

public struct Stack<Element> {

private var storage: [Element] = []

public init() { }
}

extension Stack: CustomDebugStringConvertible {

public var debugDescription: String {
"""
----top----
\(storage.map { "\(\$0)" }.reversed().joined(separator: "\n"))
-----------
"""
}

public mutating func push(_ element: Element) {
storage.append(element)
}

public mutating func pop() -> Element? {
storage.popLast()
}

public func peek() -> Element? {
storage.last
}

public var isEmpty: Bool {
peek() == nil
}

}``````
``````
func reverseStackUsingRecursion(_ st: inout Stack<Int>) {
if !st.isEmpty {
// Hold all items in Function Call Stack until we reach end of the stack
var peekElement = st.peek()
st.pop()
reverseStackUsingRecursion(&st)
// Insert all the items held in Function Call Stack one by one from the bottom
// to top. Every item is inserted at the bottom insert_at_bottom(x);
insertAtBottom(&st, peekElement!)
}
}

func insertAtBottom(_ st: inout Stack<Int>, _ item: Int) {
if st.isEmpty {
st.push(item)
} else {
// All items are held in Function Call Stack until we reach end of the stack. When the stack becomes
// empty, the st.size() becomes 0, the above if part is executed and the item is inserted at the bottom
let a = st.peek()!
st.pop()
insertAtBottom(&st, item)
// push all the items held in Function Call Stack once the item is inserted at the bottom
st.push(a)
}
}

var stackInput2 = Stack<Int>()
stackInput2.push(1)
stackInput2.push(2)
stackInput2.push(3)
stackInput2.push(4)
print("Input---->", stackInput2)// 4 3 2 1
reverseStackUsingRecursion(&stackInput2)
print("Output---->", stackInput2) // 1 2 3 4``````