两次二分，找到上下界即可

C++ 代码

class Solution {
public:
    int getNumberOfK(vector<int>& nums , int k) {
        if(nums.empty()) return 0;

        int l = 0, r = nums.size() - 1, ret = 0;
        while(l < r){//二分找下界
            int mid = l + r >> 1;
            if(nums[mid] < k) l = mid + 1;
            else r = mid;
        }
        if(nums[l] != k) return 0;

        ret = l;
        l = 0, r = nums.size() - 1;
        while(l < r){//二分找上界
            int mid = l + r + 1 >> 1;
            if(nums[mid] > k) r = mid - 1;
            else l = mid;
        }
        ret = l - ret + 1;
        return ret;
    }
};