最小的K个数

输入n个数，找出其中最小的k个数。例如输入4,5,1,6,2,7,3,8 这8个数，则最小的4个数是1,2,3,4.

解法一：O（n）的算法，只有当我们可以修改输入数组时可用

 1 int Partitoin(int* input, int low, int high)

 2 {

 3     if (input == NULL || low < 0 || high < 0 || low > high)

 4     {

 5         return -1;

 6     }

 7     int temp = input[low];

 8     while(low < high)

 9     {

10         while (high>low && input[high] >= temp)

11         {

12             high--;

13         }

14         input[low] = input[high];

15         while (low<high && input[low] <= temp)

16         {

17             low++ ;

18         }

19         input[high] = input[low];

20     }

21     input[low] = temp ;

22 

23     return low ;

24 }

25 

26 void GetLeastNumbers(int* input , int length, int k)

27 {

28     if (input == NULL || length < 1 || k < 0 || k >length)

29     {

30         return;

31     }

32     int low = 0 ;

33     int high = length - 1 ;

34     int index = Partitoin(input, low, high);

35     while (index != (k - 1))

36     {

37         if (index < (k - 1))

38         {

39             low = index + 1;

40             index = Partitoin(input, low, high);

41         }

42         else

43         {

44             high = index - 1 ;

45             index = Partitoin(input, low, high);

46         }

47     }

48     for (int i = 0 ; i < k ; i++)

49     {

50         cout<<input[i]<<" ";

51     }

52 }

解法二：O（nlogk）的算法，特别适合处理海量数据

 1 typedef  multiset<int , greater<int> > intSet ;

 2 typedef intSet::iterator  setIterator ;

 3 void GetLeastNumbers(vector<int>& data , intSet& leastNumbers, int k)

 4 {

 5     if (k < 1 || data.size() < k)

 6     {

 7         return;

 8     }

 9 

10     vector<int>::iterator ite = data.begin();

11     for (;ite != data.end() ; ite++)

12     {

13         if ( (leastNumbers.size()) < k)

14         {

15             leastNumbers.insert(*ite);

16         }

17         else  

18         {

19             setIterator iterGreatest = leastNumbers.begin() ; 

20             if (*ite < *iterGreatest)

21             {

22                 leastNumbers.erase(iterGreatest);

23                 leastNumbers.insert(*ite);

24             }

25         }

26         

27     }

28 }