自己的写法

class Solution {public:    int GetNumberOfK(vector
     
       data ,int k) {        int length = data.size();        if(length <= 0)            return 0;        for(int i = 0;i < length;i++){                    }        int index1 = GetFirst(data,k,0,length-1);        int index2 = GetLast(data,k,0,length-1);        return exit(data,k) ? (index2-index1+1) : 0;           }    int GetFirst(vector
      
        data,int k,int start,int end){        if(start == end)            return start;        int first = 0;        int mid = (start+end)/2;        if(data[mid] == k){            if(data[mid-1] != k)                return mid;            else                first = GetFirst(data,k,start,mid-1);            }        else if(data[mid] < k)            first = GetFirst(data,k,mid+1,end);        else            first = GetFirst(data,k,start,mid-1);        return first;    }    int GetLast(vector
       
         data,int k,int start,int end){        if(start == end)            return start;        int last = end;        int mid = (start+end)/2;        if(data[mid] == k){            if(data[mid+1] != k)                return mid;            else                last = GetLast(data,k,mid+1,end);            }        else if(data[mid] < k)            last = GetLast(data,k,mid+1,end);        else            last = GetLast(data,k,start,mid-1);        return last;    }    bool exit(vector
        
          data,int k){        int length = data.size();        int i = 0;        for(;i < length;i++){            if(data[i] == k)                return true;        }        return false;    }};
        
       
      
     

更简洁的代码

class Solution {public:    int GetNumberOfK(vector
     
       data ,int k) {        if(data.empty() || k <= 0)            return 0;        int FirstK = GetFirstK(data,k,0,data.size()-1);        int LastK = GetLastK(data,k,0,data.size()-1);        if(FirstK > -1 && LastK > -1)            return LastK - FirstK + 1;        else            return 0;    }    int GetFirstK(vector
      
        data,int k,int start,int end){        if(start > end)            return -1;        int mid = (end + start)/2;        if(data[mid] == k){            if(data[mid-1] == k)                end = mid - 1;            else                return mid;        }        else if(data[mid] > k)            end = mid - 1;        else if(data[mid] < k)            start = mid + 1;        return GetFirstK(data,k,start,end);    }    int GetLastK(vector
       
         data,int k,int start,int end){        if(start > end)            return -1;        int mid = (end + start)/2;        if(data[mid] == k){            if(data[mid+1] == k)                start = mid + 1;            else                return mid;        }        else if(data[mid] > k)            end = mid - 1;        else if(data[mid] < k)            start = mid + 1;        return GetLastK(data,k,start,end);            }};
       
      
     

 用循环的方法做：

注意一个问题，end只能等于mid - 1，不能等于mid，同样begin只能等于mid + 1，不能等于mid。如果换成mid，当只有两个数的时候，会陷入死循环！

class Solution {public:    int GetNumberOfK(vector
     
       data ,int k) {        int length = data.size();        if(length <= 0)            return 0;        int begin = FindFirstK(data,k);        int end = FindLastK(data,k);        if(begin != -1 && end != -1 )            return end - begin + 1;        else            return 0;    }    int FindFirstK(vector
      
        data,int k){        int length = data.size();        int begin = 0;        int end = length-1;        while(begin < end){            int mid = (begin + end)/2;            if(data[mid] == k && data[mid-1] != k)                return mid;            else if(data[mid] == k && data[mid-1] == k)                end = mid - 1;            else if(data[mid] > k)                end = mid - 1;            else if(data[mid] < k)                begin = mid + 1;        }        if(data[begin] == k)            return begin;        else            return -1;    }    int FindLastK(vector
       
         data,int k){        int length = data.size();        int begin = 0;        int end = length-1;        while(begin < end){            int mid = (begin + end)/2;            if(data[mid] == k && data[mid+1] != k)                return mid;            else if(data[mid] == k && data[mid+1] == k)                begin = mid + 1;            else if(data[mid] > k)                end = mid - 1;            else                begin = mid + 1;        }        if(data[begin] == k)            return begin;        else            return -1;    }};