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155. Min Stack

Easy

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.

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.

Solution

public class MinStack {
    private class Node {
        public int min;
        public int data;
        public Node nextNode;
        public Node previousNode;

        public Node(int min, int data, Node previousNode, Node nextNode) {
            this.min = min;
            this.data = data;
            this.previousNode = previousNode;
            this.nextNode = nextNode;
        }
    }

    private Node currentNode;

    // initialize your data structure here.
    public MinStack() {
        // no initialization needed.
    }

    public void Push(int val) {
        if (currentNode == null) {
            currentNode = new Node(val, val, null, null);
        } else {
            currentNode.nextNode = new Node(Math.Min(currentNode.min, val), val, currentNode, null);
            currentNode = currentNode.nextNode;
        }
    }

    public void Pop() {
        currentNode = currentNode.previousNode;
    }

    public int Top() {
        return currentNode.data;
    }

    public int GetMin() {
        return currentNode.min;
    }
}
}
/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.Push(val);
 * obj.Pop();
 * int param_3 = obj.Top();
 */

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