MinStack

 This article is for self practice

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 val) pushes the element val 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.

The central concept here is that each element each node on the stack has an array of two field. The first value is the actual node value and the other field is the minimum value among all the nodes below it.

package practice;

class MinStack {

    Node top= null;
    // int min=Integer.MAX_VALUE;

    public MinStack() {

    }

    public void push(int val) {
        if(top==null){
            top=new Node(val,val);
        }else{

//The central concept here is that each element each node on the stack has an array of two field. //The first value is the actual node value and the other field is the minimum value among all the //nodes below it.
            int currentMin=top.val[1];
            Node node=new Node(val, Math.min(val,currentMin));
            node.next=top;
            top=node;
        }
    }

    public void pop() {
        // if(top==null) return;        
        Node temp=top;
        top=top.next;
        temp.next=null;


    }

    public int top() {
        //assuming stack is not empty
        return top.val[0];
    }

    public int getMin() {
        return top.val[1];
    }

    class Node{
        int [] val=new int[2];
        Node next;

        Node (int val, int min ){
            this.val[0]=val;
            this.val[1]=min;
            this.next=null;
        }
    }
}


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