min stack

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.

 

思路是新建一个堆栈用来存储最小值min，每次往stack里push值的时候，如果push的值比min里保存的值小，就把新的值同时push进min里。

在pop的时候要考虑，如果要pop的值刚好是最小值，就要同时把min里的最小值pop掉。

 

java里stack的值保存不同类型，直接用==来判断会出错，需要用equals来判断是否相同。

 

 

public class MinStack {
    
    Stack<Integer> stk=new Stack<Integer>();
    Stack<Integer> min=new Stack<Integer>();
    public MinStack() {
        // do initialize if necessary
        
    }

    public void push(int number) {
        // write your code here
        if(min.empty()||number<=min.peek())
        {
            min.push(number);
        }
        stk.push(number);
    }

    public int pop() {
        // write your code here
        if(stk.peek().equals(min.peek()))
        {
            min.pop();
        }
        
        return stk.pop();
    }
    
    public int min() {
        // write your code here
        return min.peek();
    }
}