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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

class MinStack {
    private class Node(var min: Int, var data: Int, var previousNode: Node, var nextNode: Node)

    private var currentNode: Node = _

    def push(`val`: Int): Unit = {
        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
        }
    }

    def pop(): Unit = {
        currentNode = currentNode.previousNode
    }

    def top(): Int = {
        currentNode.data
    }

    def getMin(): Int = {
        currentNode.min
    }
}

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

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