[LintCode/LeetCode] Min Stack/Max Stack

Problem

Implement a stack with min() function, which will return the smallest number in the stack.

It should support push, pop and min operation all in O(1) cost.

Example

push(1)
pop() // return 1
push(2)
push(3)
min() // return 2
push(1)
min() // return 1

Note

min operation will never be called if there is no number in the stack.
注意在push()里的条件，minstack.peek() >= number，保证最小值在minstack中。

valueOf(String) returns a new java.lang.Integer, which is the object representative of the integer,
whereas parseInt(String) returns a primitive integer type int.
intValue is an instance method whereby parseInt is a static method.
Moreover, Integer.parseInt(s) can take primitive datatype as well.

Solution

Min Stack

updated 2018-9

class MinStack {

    Stack stack = new Stack<>();
    Stack minstack = new Stack<>();
    /** initialize your data structure here. */
    public MinStack() {
        
    }
    
    public void push(int x) {
        stack.push(x);
        if (minstack.isEmpty() || minstack.peek() >= x) minstack.push(x);
    }
    
    public void pop() {
        if (!stack.isEmpty() && !minstack.isEmpty() && minstack.peek().equals(stack.peek())) minstack.pop();
        stack.pop();
    }
    
    public int top() {
        if (stack.isEmpty()) return 0;
        else return stack.peek();
    }
    
    public int getMin() {
        if (minstack.isEmpty()) return 0;
        else return minstack.peek();
    }
}

Min Stack -- without using Stack class

class MinStack {

    /** initialize your data structure here. */

    private Node head;
    
    private class Node {
        int val;
        int min;
        Node next;
        private Node(int min, int val, Node next) {
            this.val = val;
            this.min = min;
            this.next = next;
        }
    }
    
    public void push(int x) {
        if (head == null) {
            head = new Node(x, x, null);
        } else {
            head = new Node(Math.min(head.min, x), x, head);
        }
    }
    
    public void pop() {
        head = head.next;
        return;
    }
    
    public int top() {
        return head.val;
    }
    
    public int getMin() {
        return head.min;
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */

MAX STACK

public void push(int x) {
    if (maxStack.isEmpty() || maxStack.peek() <= x) maxStack.push(x);
    stack.push(x);
}
public int pop() {
    if (maxStack.peek().equals(stack.peek())) maxStack.pop();
    return stack.pop();
}
public int max() {
    return maxStack.peek();
}