剑指offer 38_统计数组中k出现的个数

思路：

二分法，分别找出第一个和最后一个k出现的位置。相减 加一

#include <stdio.h>

//获取第一个K的位置
int getFirstK (int k,int *numbers,int start,int end){

    if(start > end){
        return -1;    
    }

    int middle = (start + end) / 2;
    int middleData = numbers[middle];

    if(middleData < k){
        start = middle + 1;
    }else if(middleData > k){
        end = middle - 1;
    }else{
        if(middle>0 && numbers[middle-1]!=k　　　　//注意这个if
            || middle==0){
            return middle;
        }else{
            end = middle -1;
        }
    }

    getFirstK(k,numbers,start,end);
}

//获取最后一个K的位置
int getLastK (int k,int *numbers,int start,int end){

        if(start > end){
                return -1;
        }

        int middle = (start + end) / 2;
        int middleData = numbers[middle];

        if(middleData < k){
                start = middle + 1;
        }else if(middleData > k){
                end = middle - 1;
        }else{
                if(middle<end && numbers[middle+1]!=k
                        || middle==end){
                        return middle;
                }else{
                        start = middle + 1;
                }
        }

        getLastK(k,numbers,start,end);
}

//得到k 在numbers中的个数
int getNumberOfK(int k, int* numbers,int length){
        if(numbers == NULL || length <= 0){
                return 0;
        }

        int last = getLastK(k, numbers,0, length-1);
    int first = getFirstK(k, numbers,0, length-1);

    if(last == -1 || first == -1){
        return 0;
    }
    return last - first + 1;
}


int main(){

    int numbers1[20] = {1,1,1,2,3,3,3,3,3,3,3,3,4,5,6,7,66,7787,8888,9999}; 
    printf("%d
",getNumberOfK(3,numbers1,20));

    int numbers2[10] = {1,2,2,2,2,5,6,7,8,9};
    printf("%d
",getNumberOfK(3,numbers2,10));

    int* numbers3 = NULL;
    printf("%d
",getNumberOfK(3,numbers3,10));

}