(二分查找) $O(log（n）)$

C++ 代码

class Solution {
public:
    int getNumberOfK(vector<int>& nums , int k) {
       int half = (nums.size()-1)/2;
       int r = nums.size()-1;
       int l = 0;
       if (nums.empty()) return 0;

       while(l<=r){
           half = (l+r)/2;
           if (nums[half]<k){
               l = half + 1;
           }
           else if (nums[half] >= k){
               r = half-1;
           }
       }
       int cnt = 0;
       int a = half+1;
       int b = half;
    //   cout<<nums[half]<<endl;
    //   cout<<half<<endl;
      while(k == nums[a] && a<nums.size() ) {
        a++;
        cnt++;
        // cout<<cnt<<endl;
      }
      while(k == nums[b] && b>=0){
         b--;
         cnt++;
      }
       return cnt;
    }
};