Min Stack

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. MinStack minStack = new MinStack();
2. minStack.push(-2);
3. minStack.push(0);
4. minStack.push(-3);
5. minStack.getMin(); --> Returns -3.
6. minStack.pop();
7. minStack.top(); --> Returns 0.
8. minStack.getMin(); --> Returns -2.

Solution:
1. class MinStack {
2. Stack s1 = new Stack<>();
3. Stack s2 = new Stack<>();
4. 5. public void push(int x) {
6. s1.push(x);
7. if (s2.isEmpty() || x < s2.peek()[0]) {
8. s2.push(new Integer[]{x, 1});
9. } else if (x == s2.peek()[0]) {
10. s2.peek()[1]++;
11. }
12. }
13. 14. public void pop() {
15. if (s1.peek().intValue() == s2.peek()[0]) {
16. s2.peek()[1]--;
17. if (s2.peek()[1] == 0)
18. s2.pop();
19. }
20. s1.pop();
21. }
22. 23. public int top() {
24. return s1.peek();
25. }
26. 27. public int getMin() {
28. return s2.peek()[0];
29. }
30. }