LeetCode(64)- Min Stack

题目：

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
getMin() -- Retrieve the minimum element in the stack.

思路：

代码：

class MinStack {
Stack<Integer> stack = new Stack<Integer>();
Stack<Integer> min = new Stack<Integer>();
public void push(int x) {
if(min.isEmpty() || x <= min.peek()){
min.push(x);
}
stack.push(x);
}

public void pop() {
if(stack == null){
return;
}
if(min.peek().equals(stack.peek())){
min.pop();
}
stack.pop();
}

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

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