Min Stack

Implementastackwithmin()function,whichwillreturnthesmallestnumberinthestack.

Itshouldsupportpush,popandminoperationallinO(1)cost.

Notice

minoperationwillneverbecalledifthereisnonumberinthestack.

Example

push(1)
pop()//return1
push(2)
push(3)
min()//return2
push(1)
min()//return1

分析：
用另一个stack来保存当前最小的值。

publicclassMinStack{

Stack<Integer>elements=newStack<Integer>();
Stack<Integer>minStack=newStack<Integer>();

publicMinStack(){
//doinitializeifnecessary
}

publicvoidpush(intx){
elements.push(x);
if(minStack.isEmpty()||x<=minStack.peek()){
minStack.push(x);
}
}

publicintpop(){
if(elements.isEmpty()){
return-1;
}
//这个地方太蛋疼了，居然要用equals...
if(elements.peek().equals(minStack.peek())){
minStack.pop();
}
returnelements.pop();
}

publicintmin(){
returnminStack.peek();
}
}