0155-Min-Stack.py

'''
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.
'''      

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

# Solution by keeping track of miniumum node at each element

class MinStack:

    def __init__(self):
        self.stack = []

    def push(self, val: int) -> None:
        if self.stack:
            self.stack.append([val, min(val, self.getMin())])
        else:
            self.stack.append([val, val]) 

    def pop(self) -> None:
        return self.stack.pop() if self.stack else None

    def top(self) -> int:
        return self.stack[-1][0] if self.stack else None

    def getMin(self) -> int:
        return self.stack[-1][1] if self.stack else None

# Solution by initialising another stack to track minimum element

class MinStack:

    def __init__(self):
        self.stack = []
        self.min_stack = []

    def push(self, x: int) -> None:        
        self.stack.append(x)        
        if not self.min_stack or x <= self.min_stack[-1]:
            self.min_stack.append(x)

    def pop(self) -> None:
        if self.stack[-1] == self.min_stack[-1]:
            self.min_stack.pop()        
        self.stack.pop()
        
    def top(self) -> int:
        return self.stack[-1]

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