15-155. Min Stack

题目描述：

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.

代码：

 1 class MinStack(object):
 2 
 3     def __init__(self):
 4         """
 5         initialize your data structure here.
 6         """
 7         self.stack = [] # 如果这个stack想在整个class中都可以使用，则必须要加上self.
 8 
 9     def push(self, x):
10         """
11         :type x: int
12         :rtype: None
13         """
14         curMin = self.getMin()
15         if curMin == None or x < curMin:
16             curMin = x
17         self.stack.append((x, curMin))
18 
19     def pop(self):
20         """
21         :rtype: None
22         """
23         self.stack.pop()
24 
25     def top(self):
26         """
27         :rtype: int
28         """
29         return self.stack[-1][0] if self.stack else None
30 
31     def getMin(self):
32         """
33         :rtype: int
34         """
35         return self.stack[-1][1] if self.stack else None
36 
37 
38 # Your MinStack object will be instantiated and called as such:
39 # obj = MinStack()
40 # obj.push(x)
41 # obj.pop()
42 # param_3 = obj.top()
43 # param_4 = obj.getMin()

 

分析：

1. python的栈、链表、树都是用list实现的。

2. 如果一个在class内部定义的变量想要在整个类中都可以使用，则这个变量的前面必须加上self. 。

posted @ 2019-06-03 23:52  mingyu02  阅读(132)  评论(0)  编辑  收藏  举报