# 155. Min Stack

## 1. Question

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.

## 2. Examples

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


## 3. 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.

## 5. Solutions

class MinStack {

Deque<Integer> input;
Deque<Integer> sorted;

public MinStack() {
}

public void push(int val) {
input.push(val);

if (sorted.isEmpty() || sorted.peek() >= val) {
sorted.push(val);
}

}

public void pop() {
if (sorted.peek().equals(input.pop())) {
sorted.pop();
}

}

public int top() {
return input.peek();
}

public int getMin() {
return sorted.peek();
}
}