題目

　　統計一個數字在排序數組中出現的次數。

分析

　　利用二分查找，找到該數字第一次出現的位置和最後一次出現的位置。

代碼

 1     public int GetNumberOfK(int[] array, int k){
 2         int first = GetFirst(array, k, 0, array.length-1);
 3         int last = GetLast(array, k, 0, array.length-1);
 4         if(first==-1 || last==-1)
 5             return
 
 0;
 6         return last-first+1;
 7     }
 8     
 9     public int GetFirst(int[] array, int k, int low, int high){
10         int tag = -1;
11         
12         while(low<=high){
13             int mid = (low+high)/2;
14             if(array[mid]>k){
15                 high = mid-1;
16             }
 
17             else if(array[mid]<k){
18                 low = mid+1;
19             }
20             else{
21                 if(mid==0 || (mid>0 && (array[mid-1]<array[mid]))){
22                     tag = mid;
23                     break;
24                 }
25                 else
 
{
26                     high = mid-1;
27                 }
28             }
29         }
30         return tag;
31     }
32     
33     public int GetLast(int[] array, int k, int low, int high){
34         int tag = -1;
35         while(low<=high){
36             int mid = (low+high)/2;
37             if(array[mid]>k)
38                 high = mid-1;
39             else if(array[mid]<k)
40                 low = mid+1;
41             else{
42                 if(mid==array.length-1 || (mid<array.length-1 && (array[mid]<array[mid+1]))){
43                     tag = mid;
44                     break;
45                 }
46                 else{
47                     low = mid+1;
48                 }
49             }
50         }
51         return tag;
52     }

數字在排序數組中出現的次數