0155. Min Stack

https://leetcode.com/problems/min-stack

Description

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.

ac1: two stack

class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> stack2;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        stack2 = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);
        if (stack2.isEmpty() || x <= stack2.peek()) {
            stack2.push(x);
        }
    }

    public void pop() {
        int tmp = stack.pop();
        if (tmp == stack2.peek()) {
            stack2.pop(); 
        }
    }

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

    public int getMin() {
        return stack2.peek();
    }
}

ac2: one stack

Key: in the stack, add some roadmark to indicate min from now on.ß

class MinStack {
    private Stack<Integer> stack;
    private int min;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        min = Integer.MAX_VALUE;
    }

    public void push(int x) {
        if (x <= min) {
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }

    public void pop() {
        int tmp = stack.pop();
        if (tmp == min) {
            min = stack.pop();
        }
    }

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

    public int getMin() {
        return min;
    }
}