leetcode-155-min-stack.swift

/**

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https://leetcode.com/problems/min-stack/


Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
getMin() -- Retrieve the minimum element in the stack.
 

Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

*/

/**
Big O Annotation
Time complexity O(1) all operations are constant.
Space complexity O(n) where n is the number of elements in the stack.
*/
class MinStack {

    /**
    Min stack holds all min elements
    with the current lowest min on the top (last)
    */
    private var minStack: [Int] = []

    // Array of all elements on the stack
    private var array: [Int] = []

    // Empty init
    init() {}
    
    func push(_ x: Int) {

        // Add element to general stack    
        array.append(x)
        
        // Check if we have a last min element
        guard let last = minStack.last else {

            // If not add the current element
            minStack.append(x)
            return 
        }

        /**
        Otherwise check if we have a new minimum element.
        
        Initially I was setting this just to ">"m
        but I failed a test that expects same minimum
        elements to also be stored/returned
        */
        if last >= x {
            minStack.append(x)
        }
    }
    
    func pop() {
        
        // Pop the last element from the general stack
        let popped = array.popLast()
        
        /**
        Check if it's equal to our last element of the 
        min stack, if so pop it as well
        */
        if let last = minStack.last, last == popped {
            minStack.popLast()
        }
    }
    
    func top() -> Int {

        /**
        "Peeks" on the last element of the general array.
        
        There was no requirement for what to return if 
        there is no last element so I'm returning -1
        as this follows the requirements of other challenges
        on leetcode.com.
        */
        return array.last ?? -1
    }
    
    func getMin() -> Int {

        /**
        "Peeks" on the last element of the min stack array.
        
        There was no requirement for what to return if 
        there is no last element so I'm returning -1
        as this follows the requirements of other challenges
        on leetcode.com.
        */
        return minStack.last ?? -1
    }
}