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 <= 2^{31 - 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.

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/min-stack 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

class MinStack {

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

  public MinStack() {
    input = new LinkedList<>();
    sorted = new LinkedList<>();
  }

  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();
  }
}