最大子序列和问题

int MaxSub(int *a, int n)
{
    int sum = 0;
    int tmp;
 
    for (int i = 0; i < n; i++)
    {
        for (int j = i; j < n; j++)
        {
            tmp = 0;
            for (int k = i; k <= j; k++)
                tmp += a[k];
 
            if (tmp > sum)
             sum = tmp;
        }
    }
    return sum;
}

int MaxSub(int *a, int n)
{
    int sum = 0;
    int tmp = 0;
 
    for (int i = 0; i < n; i++)
    {
        for (int j = i; j < n; j++)
        {
            tmp += a[j];
            if (tmp > sum)
             sum = tmp;
        }
        tmp = 0;
    }
    return sum;
}

int MaxSub(int *a, int left, int right)
{
    // 这里的基准情况是关键
    if (left == right)
    {
        if (a[left] > 0)
            return a[left];
        else
            return 0;
    }
 
    int middle = (left+right)/2;
    int leftmax = MaxSub(a, left, middle);
    int rightmax = MaxSub(a, middle+1, right);
 
    int tmp = 0;
    int leftbordersum = 0;
    for (int i = middle; i >= left; i--)
    {
        tmp += a[i];
        if (tmp > leftbordersum)
            leftbordersum = tmp;
    }
 
    tmp = 0;
    int rightbordersum = 0;
    for (int i = middle+1; i <= right; i++)
    {
        tmp += a[i];
        if (tmp > rightbordersum)
            rightbordersum = tmp;
    }
 
    int bordersum = leftbordersum + rightbordersum;
 
    return max3(leftmax, rightmax, bordersum);
}

int MaxSub(int *a, int n)
{
    int tmp = 0;
    int sum = 0;
 
    for (int i = 0; i < n; i++)
    {
        tmp += a[i];
        if (tmp > sum)
            sum = tmp;
        else if (tmp < 0)
            tmp = 0;
    }
    return sum;
}