# 155. 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

Approach 1: Two stacks.

# Code (Python)

Approach 1:

class MinStack:

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.nums = []
        self.min_vals = []

    def push(self, x: int) -> None:
        self.nums.append(x)
        if self.min_vals:
            self.min_vals.append(min(self.min_vals[-1], x))
        else:
            self.min_vals.append(x)

    def pop(self) -> None:
        if self.nums:
            self.nums.pop()
            self.min_vals.pop()

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

    def getMin(self) -> int:
        if self.nums:
            return self.min_vals[-1]

# Code (C++)

Approach 1:

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {
    }
    
    void push(int x) {
        dataStack.push(x);
        if (minStack.empty() || x <= minStack.top())
            minStack.push(x);
    }
    
    void pop() {
        if (dataStack.top() == minStack.top())
            minStack.pop();
        dataStack.pop();
    }
    
    int top() {
        return dataStack.top();
    }
    
    int getMin() {
        return minStack.top();
    }

private:
    stack<int> dataStack;
    stack<int> minStack;
};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */