155. Min Stack

1. Description

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.

2. Example

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

3. Constraints

• Methods pop, top and getMin operations will always be called on non-empty stacks.

4. Solutions

My Accepted Solution

n is the number of nodes
Time complexity: O(n)
Space complexity: O(n)

class MinStack 
{
private:
    stack<int> storage;
    stack<int> minValueStack;
public:
    
    MinStack() 
    {
        
    }
    
    void push(int x) 
    {
        storage.push(x);
        
        if(minValueStack.empty() || x <= minValueStack.top())
        {
            minValueStack.push(x);
        }
    }
    
    void pop() 
    { 
        if(!minValueStack.empty() && storage.top() == minValueStack.top())
        {
            minValueStack.pop();
        }
            
        storage.pop();    
    }
    
    int top() 
    {
        return storage.top();
    }
    
    int getMin() 
    {
        return minValueStack.top();
    }
};