LeetCode-in-All

155. Min Stack

Easy

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.

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:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

Solution

(define min-stack%
  (class object%
    (super-new)
    
    (define vals '())
    
    ; push : exact-integer? -> void?
    (define/public (push val)
        (let* ([cur-min (if (empty? vals) val (get-min))]
               [new-top (cons val (min cur-min val))])
            (set! vals (cons new-top vals))))

    ; pop : -> void?
    (define/public (pop)
      (define res (top))
      (set! vals (cdr vals))
      res)

    ; top : -> exact-integer?
    (define/public (top)
      (car (first vals)))

    ; get-min : -> exact-integer?
    (define/public (get-min)
      (cdr (first vals)))))

;; Your min-stack% object will be instantiated and called as such:
;; (define obj (new min-stack%))
;; (send obj push val)
;; (send obj pop)
;; (define param_3 (send obj top))
;; (define param_4 (send obj get-min))

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