0155-Min Stack.md

Min Stack

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

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

You must implement a solution with O(1) time complexity for each function.

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

Constraints:

• -pow(2, 31) <= val <= pow(2, 31) - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * pow(10, 4) calls will be made to push, pop, top, and getMin.

Solution

extension MinStack {
    struct Element {
        let val: Int
        let min: Int
    }
}

class MinStack {
    private var stack: [Element]
    private var last: Element {
        if let last = stack.last {
            return last
        } else {
            fatalError()
        }
    }

    init() {
        stack = []
    }
    
    func push(_ val: Int) {
        let element: Element
        if let last = stack.last, last.min < val {
            element = Element(val: val, min: last.min)
        } else {
            element = Element(val: val, min: val)
        }
        stack.append(element)
    }
    
    func pop() {
        if stack.isEmpty {
            fatalError()
        } else {
            _ = stack.removeLast()
        }
    }
    
    func top() -> Int {
        last.val
    }
    
    func getMin() -> Int {
        last.min
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * let obj = MinStack()
 * obj.push(val)
 * obj.pop()
 * let ret_3: Int = obj.top()
 * let ret_4: Int = obj.getMin()
 */