Soyo

Day 10 of 30 days Leetcode challenge

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:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

Solution

struct MinNode {
    let val: Int
    let min: Int
}

class MinStack {

    private var stack: [MinNode] = []

    /** initialize your data structure here. */
    init() {
        
    }
    
    func push(_ x: Int) {
        guard !stack.isEmpty else {
            stack.append(MinNode(val: x, min: x))
            return 
        }
        
        let top = self.stack.last!
        stack.append(MinNode(val: x, min: min(top.min, x)))
    }
    
    func pop() {
        stack.removeLast()
    }
    
    func top() -> Int {
        return stack.last!.val
    }
    
    func getMin() -> Int {
        return stack.last!.min
    }
}

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