序列递增：二分性质

class Solution {
public:
    int getNumberOfK(vector<int>& nums , int k) {
        if(nums.size() == 0)    return 0;
        int l = 0, r = nums.size() - 1;
        while(l < r){
            int mid = l + r >> 1;
            if(nums[mid] >= k)  r = mid;
            else    l = mid + 1;
        }
        if(nums[l] != k)    return 0;
        int cnt = 0;
        for(int i = l; i <= nums.size(); i ++ ){
            if(nums[i] == k)    cnt ++ ;
            else    break;
        }
        return cnt;
    }
};