LeetCode 155. Min Stack

原题链接在这里: https://leetcode.com/problems/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.

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.

题解:

可以用另一个stack, min_stk来维护最小值. push()时检测x是否小于等于min_stk 的peek(), 若true,则同时push进min_stk.

pop()时若该值等于min_stk.peek()则同时对min_stk进行pop()操作。

getMin()返回min_stk.peek()即可。

Note:使用peek(), pop()之前,都要检测stack.isEmpty(), 否则会throw exception.

Time Complexity: getMin(), O(1); push(), O(1); pop(), O(1); top(), O(1).

Space: O(n).

AC Java:

 1 public class MinStack {
 2 
 3     Stack<Integer> stk;
 4     Stack<Integer> minStk;
 5     public MinStack() {
 6         stk = new Stack<Integer>();
 7         minStk = new Stack<Integer>();
 8     }
 9     
10     public void push(int x) {
11         stk.push(x);
12         if(minStk.isEmpty() || minStk.peek()>=x){
13             minStk.push(x);
14         }
15     }
16     
17     public void pop() {
18         if(!stk.isEmpty()){
19             int x = stk.pop();
20             if(!minStk.isEmpty() && x == minStk.peek()){
21                 minStk.pop();
22             }
23         }
24     }
25     
26     public int top() {
27         if(!stk.isEmpty()){
28             return stk.peek();
29         }
30         throw new IllegalStateException("Stack is empty!");
31     }
32     
33     public int getMin() {
34         if(!minStk.isEmpty()){
35             return minStk.peek();
36         }
37         throw new IllegalStateException("Stack is empty!");
38     }
39 }
40 
41 /**
42  * Your MinStack object will be instantiated and called as such:
43  * MinStack obj = new MinStack();
44  * obj.push(x);
45  * obj.pop();
46  * int param_3 = obj.top();
47  * int param_4 = obj.getMin();
48  */

类似Max Stack.

posted @ 2015-08-19 05:07  Dylan_Java_NYC  阅读(285)  评论(0编辑  收藏  举报