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.

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.

 class ListNode:
    def __init__(self, x, min = None):
        self.val = x
        self.next = None
        self.min = min

class MinStack:

    def __init__(self):
        self.head = None
        self.min = None


    def push(self, value: int) -> None:
        if not self.head:
            self.head = ListNode(value, value)
            self.min = value
        else:
            min = value if value < self.min else self.min
            self.min = min
            node = ListNode(value,  min)
            node.next = self.head
            #self.head.next = node
            self.head = node

    def pop(self) -> None:
        if self.head is not None:
            self.head = self.head.next
            if self.head is not None:
                self.min = self.head.min
            else:
                self.min = None

    def top(self) -> int:
        if self.head is not None:
            return self.head.val


    def getMin(self) -> int:
        return self.min


# Your MinStack object will be instantiated and called as such:
# obj = MinStack()
# obj.push(val)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.getMin()

Min Stack

Random Note