MinStack.java

package Leetcode;

/**
 * @author kalpak
 *
 * 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 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:
 *
 * Methods pop, top and getMin operations will always be called on non-empty stacks.
 */

class MinStack {
 private Node head;

 /** initialize your data structure here. */
 public MinStack() {

 }

 public void push(int x) {
 if(head == null)
 head = new Node(x, x);
 else
 head = new Node(x, Math.min(head.minVal, x), head);
 }

 public void pop() {
 head = head.next;
 }

 public int top() {
 return head.val;
 }

 public int getMin() {
 return head.minVal;
 }

 class Node {
 int val;
 int minVal;
 Node next;

 public Node(int val, int minVal) {
 this.val = val;
 this.minVal = minVal;
 next = null;
 }

 public Node(int val, int minVal, Node next) {
 this.val = val;
 this.minVal = minVal;
 this.next = next;
 }
 }
}