二分查找

int binary_search(int* a, int len, int goal)
{
    int low = 0;
    int high = len - 1;
    while(low <= high)
    {
        int middle = (low + high)/2;
        if(a[middle] == goal)
            return middle;
        //在左半边
        else if(a[middle] > goal)
            high = middle - 1;
        //在右半边
        else
            low = middle + 1;
    }
    //没找到
    return -1;
}