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.

本题我创建了一个栈，每次都压入了min，代码如下：
 1 public class MinStack {
 2     Stack<Integer> s;
 3     int min = Integer.MAX_VALUE;
 4     /** initialize your data structure here. */
 5     public MinStack() {
 6         s = new Stack<Integer>();
 7         s.push(min);
 8     }
 9     
10     public void push(int x) {
11         min = s.peek();
12         if(min>x) min = x;
13         s.push(x);
14         s.push(min);
15     }
16     
17     public void pop() {
18         s.pop();
19         min = s.pop();
20     }
21     
22     public int top() {
23         min=s.pop();
24         int top = s.peek();
25         s.push(min);
26         return top;
27     }
28     
29     public int getMin() {
30         return s.peek();
31     }
32 }
33 
34 /**
35  * Your MinStack object will be instantiated and called as such:
36  * MinStack obj = new MinStack();
37  * obj.push(x);
38  * obj.pop();
39  * int param_3 = obj.top();
40  * int param_4 = obj.getMin();
41  */

后来看了答案，发现其实不用占用那么多的内存，代码如下：

 1 public class MinStack {
 2     Stack<Integer> s;
 3     int min;
 4     /** initialize your data structure here. */
 5     public MinStack() {
 6         s = new Stack<Integer>();
 7         min = Integer.MAX_VALUE;
 8     }
 9     
10     public void push(int x) {
11         if(x<=min){
12             s.push(min);
13             min = x;
14         }
15         s.push(x);
16     }
17     
18     public void pop() {
19         if(s.pop()==min){
20             min = s.pop();
21         }
22     }
23     
24     public int top() {
25         return s.peek();
26     }
27     
28     public int getMin() {
29         return min;
30     }
31 }
32 
33 /**
34  * Your MinStack object will be instantiated and called as such:
35  * MinStack obj = new MinStack();
36  * obj.push(x);
37  * obj.pop();
38  * int param_3 = obj.top();
39  * int param_4 = obj.getMin();
40  */

 

posted @ 2017-03-04 08:52  CodesKiller  阅读(176)  评论(0)  编辑  收藏  举报