数字在排序数组中出现的次数

题目：统计一个数字在排序数组中出现的次数。例如输入排序数组{1，2，3，3，3，3，4，5}和数字3，由于3在这个数组中出现了4次，因此输出4.

找到排序数组中的第一个K：

int GetFirstK(int *data, int length, int k, int start, int end)
{
    if(start > end)
        return -1;
    
    int middleIndex = start + ((end - start) >> 1);
    int middleData = data[middleIndex];
    
    if(middleData == k)
    {
        if((middleIndex > 0 && data[middleIndex - 1] != k) || middleIndex == 0)
            return middleIndex;
        else
            end = middleIndex - 1;
    }
    else if(middleData > k)
        end = middleIndex - 1;
    else
        start = middleIndex + 1;
    
    return GetFirstK(data, length, k, start, end);
}

 找到排序数组中最后一个k：

int GetLastK(int *data, int length, int k, int start, int end)
{
    if(start > end)
        return -1;
    
    int middleIndex = start + ((end - start) >> 1);
    int middleData = data[middleIndex];
    
    if(middleData == k)
    {
        if((middleIndex < length - 1 && data[middleIndex + 1] != k) || middleIndex == length -1)
            return middleIndex;
        else
            start = middleIndex + 1;
    }
    else if(middleData < k)
        start = middleIndex + 1;
    else
        end = middleIndex - 1;
    
    return GetLastK(data, length, k, start, end);
}

在分别找到第一个k和最后一个k的下标之后，就能计算出k在数组中出现的次数了。相应的代码如下：

int GetNumberOfK(int *data, int length, int k)
{
    int number = 0;
    
    if(data != NULL && length > 0)
    {
        int first = GetFirstK(data, length, k, 0, length - 1);
        int last = GetLastK(data, length, k, 0, length - 1);
        
        if(first > -1 && last > -1)
            number = last - first + 1;
    }
    
    return number;
}

完整代码如下：

int GetFirstK(int *data, int length, int k, int start, int end)
{
    if(start > end)
        return -1;
    
    int middleIndex = start + ((end - start) >> 1);
    int middleData = data[middleIndex];
    
    if(middleData == k)
    {
        if((middleIndex > 0 && data[middleIndex - 1] != k) || middleIndex == 0)
            return middleIndex;
        else
            end = middleIndex - 1;
    }
    else if(middleData > k)
        end = middleIndex - 1;
    else
        start = middleIndex + 1;
    
    return GetFirstK(data, length, k, start, end);
}

int GetLastK(int *data, int length, int k, int start, int end)
{
    if(start > end)
        return -1;
    
    int middleIndex = start + ((end - start) >> 1);
    int middleData = data[middleIndex];
    
    if(middleData == k)
    {
        if((middleIndex < length - 1 && data[middleIndex + 1] != k) || middleIndex == length -1)
            return middleIndex;
        else
            start = middleIndex + 1;
    }
    else if(middleData < k)
        start = middleIndex + 1;
    else
        end = middleIndex - 1;
    
    return GetLastK(data, length, k, start, end);
}

int GetNumberOfK(int *data, int length, int k)
{
    int number = 0;
    
    if(data != NULL && length > 0)
    {
        int first = GetFirstK(data, length, k, 0, length - 1);
        int last = GetLastK(data, length, k, 0, length - 1);
        
        if(first > -1 && last > -1)
            number = last - first + 1;
    }
    
    return number;
}