先查找该数在在最左边的位置,再查找该数在最右边的位置

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

        while(l<r){ //模板1,将[l, r]分为[l, mid]和[mid+1, r]
            int mid = l+r>>1;
            if(nums[mid]<k)  l = mid+1;
            else    r = mid;
        }

        int left = l;
        if(nums[left]!=k)   return 0;   //没有找到k
        // printf("%d\n",left);
        l=0, r = nums.size()-1;
        while(l<r){ //模板2,将[l, r]分为[l,mid-1]和[mid, r]
            int mid = l+r+1>>1;
            if(nums[mid]<=k)   l=mid;
            else r = mid-1;
        }
        // printf("%d\n",r);

        return r-left+1;
    }
};