0155-min-stack

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Description

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 value) pushes the element value 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:

  • -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(Python)

class MinStack:
    # -2 0 -3  -3 
    # data structure 
    #  stack = [] holds stack data
    #  minimum stack = stack of (minimum number,  count)
    def __init__(self):
        self.stk = []
        self.min_stack = [] # min stack with tracker to avoid duplicates

    def push(self, value: int) -> None:
        self.stk.append(value)
        if not  self.min_stack  or value <  self.min_stack[-1][0]:
            self.min_stack.append([value, 1])
        elif value == self.min_stack[-1][0]:
            self.min_stack[-1][1] += 1

    def pop(self) -> None:
        if self.min_stack[-1][0] == self.stk[-1]:
            self.min_stack[-1][1] -= 1
        if self.min_stack[-1][1] == 0:
            self.min_stack.pop()
        self.stk.pop()


    def top(self) -> int:
        return self.stk[-1]

    def getMin(self) -> int:
        return self.min_stack[-1][0]
        


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