155. Min Stack

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.

  Example:

  MinStack minStack = new MinStack();
  minStack.push(-2);
  minStack.push(0);
  minStack.push(-3);
  minStack.getMin();   --> Returns -3.
  minStack.pop();
  minStack.top();      --> Returns 0.
  minStack.getMin();   --> Returns -2.

• 思路：可以用stack自己的函数实现push，pop，top这三个，关键就是getmin不好实现，那么需要另外一个minstack来存储小的数字。

  public class MinStack {
  
      /** initialize your data structure here. */
      
     	private Stack<Integer>stack;
  	private Stack<Integer>minStack;
  	public  MinStack() {
          stack=new Stack<>();
          minStack=new Stack<>();
      }
      
      public void push(int x) {//入栈
          stack.push(x);
          if(minStack.isEmpty()||x<=minStack.peek())//如果为空的或者是入栈的元素不大于栈顶元素均可入minstack。
          {
          	minStack.push(x);
          }
      }
      
      public void pop() {
         if(stack.peek().equals(minStack.peek()))
      	{
      		minStack.pop();//保证如果某个元素要出栈的时候，刚好在minstack里面，那么出栈。
      	}
          stack.pop();     
      }
      
      public int top() {
          return stack.peek();
      }
      
      public int getMin() {
      	return minStack.peek(); //因为minstack的栈顶元素永远都是当前栈中所有元素中的最小值，那么直接出栈顶元素即可。      
      }
  }