数字在排序数组中出现的次数

Posted 2020-10-22 Jeysin

题目描述

统计一个数字在排序数组中出现的次数

 

思路：用二分法分别查找数字在数组中出现的开头和结尾的位置，次数=结尾-开头+1，时间复杂度为O(logn)

 1 class Solution {
 2 public:
 3     int getFirstK(vector<int> &data, int k)
 4     {
 5         int begin=0;
 6         int end=data.size()-1;
 7         int mid;
 8         while(begin<=end)
 9         {
10             mid=(begin+end)/2;
11             if(data[mid]==k && mid==0)return mid;
12             else if(data[mid]==k && data[mid-1]!=k)return mid;
13             else if(data[mid]>=k)end=mid-1;
14             else begin=mid+1;
15         }
16         return -1;
17     }
18     int getLastK(vector<int> &data, int k)
19     {
20         int begin=0;
21         int end=data.size()-1;
22         int mid;
23         while(begin<=end)
24         {
25             mid=(begin+end)/2;
26             if(data[mid]==k && mid==data.size()-1)return mid;
27             else if(data[mid]==k && data[mid+1]!=k)return mid;
28             else if(data[mid]<=k)begin=mid+1;
29             else end=mid-1;
30         }
31         return -1;
32     }
33     int GetNumberOfK(vector<int> data ,int k) {
34         if(data.size()==0)return 0;
35         int first=getFirstK(data, k);
36         int last=getLastK(data, k);
37         if(first==-1 || last==-1)return 0;
38         return last-first+1;
39     }
40 };