LeetCode: Min Stack 解题报告

Min Stack

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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.

 

 

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SOLUTION 1:

比较直观。用一个min stack专门存放最小值，如果有比它小 或是相等的（有多个平行的最小值都要单独存放，否则pop后会出问题），

则存放其到minstack.具体看代码：

 1 class MinStack {

 2     Stack<Integer> elements = new Stack<Integer>();

 3     Stack<Integer> minStack = new Stack<Integer>();

 4     

 5     public void push(int x) {

 6         elements.push(x);

 7         if (minStack.isEmpty() || x <= minStack.peek()) {

 8             minStack.push(x);

 9         }

10     }

11 

12     public void pop() {

13         if (elements.isEmpty()) {

14             return;

15         }

16         

17         // 这个地方太蛋疼了，居然要用equals...

18         if (elements.peek().equals(minStack.peek())) {

19             minStack.pop();

20         }

21         elements.pop();

22     }

23 

24     public int top() {

25         return elements.peek();       

26     }

27 

28     public int getMin() {

29         return minStack.peek();

30     }

31 }

View Code

 2014.1229 redo.

 1 class MinStack {

 2     Stack<Integer> s = new Stack<Integer>();

 3     Stack<Integer> min = new Stack<Integer>();

 4     

 5     public void push(int x) {

 6         s.push(x);

 7         if (min.isEmpty() || x <= min.peek()) {

 8             min.push(x);

 9         }

10     }

11     

12     // Pop 1: use equals.

13     public void pop1() {

14         // BUG 1: Very very trick. we should use EQUALS here instead of  "=="

15         if (s.peek().equals(min.peek())) {

16             min.pop();

17         }

18         s.pop();

19     }

20     

21     // Pop 2: use int

22     public void pop2() {

23         // BUG 1: Very very trick. we should use EQUALS here instead of  "=="

24         int n1 = s.peek();

25         int n2 = min.peek();

26         if (n1 == n2) {

27             min.pop();

28         }

29         s.pop();

30     }

31     

32     // Pop 3: use (int)

33     public void pop() {

34         // BUG 1: Very very trick. we should use EQUALS here instead of  "=="

35         if ((int)s.peek() == (int)min.peek()) {

36             min.pop();

37         }

38         s.pop();

39     }

40 

41     public int top() {

42         return s.peek();

43     }

44 

45     public int getMin() {

46         return min.peek();

47     }

48 }

View Code

 

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/stack/MinStack.java