155. Min Stack

Min Stack 

https://www.youtube.com/watch?v=8Ub73n4ySYk&t=25s

// use two stack . Compile error 
class MinStack {
   Stack<Integer> stack;
   Stack<Integer> min; 

    /** initialize your data structure here. */
    public MinStack() {
       stack = new Stack<>();
       min = new Stack<>();
        
    }
    
    public void push(int x) {
      if(stack.isEmpty()){
        min.push(x);
      }else{
        if(min.peek() >= x){
          min.push(x);
        }
      }
      stack.push(x);
    }
        
    
      
      
    public void pop() {
      int x = stack.pop();
      if(x == min.peek()){
        min.pop();
      }  
    }
    
    public int top() {
      return stack.peek();
        
    }
    
    public int getMin() {
      return min.peek();
        
    }
}


// use one stack , but keep two information on  each stack level 

 

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.



use only one stack 

The question is ask to construct One stack. So I am using one stack.

The idea is to store the gap between the min value and the current value;

The problem for my solution is the cast. I have no idea to avoid the cast. Since the possible gap between the current value and the min value could be Integer.MAX_VALUE-Integer.MIN_VALUE;

public class MinStack {
    long min;
    Stack<Long> stack;

    public MinStack(){
        stack=new Stack<>();
    }
    
    public void push(int x) {
        if (stack.isEmpty()){
            stack.push(0L);
            min=x;
        }else{
            stack.push(x-min);//Could be negative if min value needs to change
            if (x<min) min=x;
        }
    }

    public void pop() {
        if (stack.isEmpty()) return;
        
        long pop=stack.pop();
        
        if (pop<0)  min=min-pop;//If negative, increase the min value
        
    }

    public int top() {
        long top=stack.peek();
        if (top>0){
            return (int)(top+min);
        }else{
           return (int)(min);
        }
    }

    public int getMin() {
        return (int)min;
    }
}