用cin或scanf输入30万组数字

Question

Hi,

I am participating in programming contest. My algorithm is fine with number of sets to 5000 . Sets of values are consist of three integers. But I enter 300 000 sets of numbers, it takes too long.

Limit of running program: 14s . Fetching data: 576s . (Way too long)

My formatted input is:

300000
a b c

300000 - number of sets a, b, c - elements of the set

My algorithm (dont judge about the code):

#include <iostream>

using namespace std;
int min_replacements(int n, int *ds, int *ps, int *rs);
int max(int a, int b, int c);
bool ot(int a, int b, int c);
bool ooo(int a, int b, int c);
bool to(int a, int b, int c);
int main()
{
    int n = 0;
    cin >> n;
    int *ds, *ps, *rs;
    ds = new int[n];
    ps = new int[n];
    rs = new int[n];
    int d{}, p{}, r{};
    for (int i = 0; i < n; i++)
    {
        scanf("%d %d %d", &ds[i], &ps[i], &rs[i]);
        printf("%d", i);
    }
    int t = min_replacements(n, ds, ps, rs);
    printf("%d\n", t);
    delete[] ds;
    delete[] ps;
    delete[] rs;
}
bool ot(int a, int b, int c)
{
    return (a != 0 && b == 0 && c == 0);
}
bool ooo(int a, int b, int c)
{
    return (a == 0 && b != 0 && c == 0);
}
bool to(int a, int b, int c)
{
    return (a == 0 && b == 0 && c != 0);
}
int max(int a, int b, int c)
{
    int m = 0;
    if (a == b && c < a)
    {
        m = a;
    }
    if (b == c && a < b)
    {
        m = b;
    }
    if (a == c && b < c)
    {
        m = c;
    }
    if (b < a && c < a)
    {
        m = a;
    }
    if (a < b && c < b)
    {
        m = b;
    }
    if (a < c && b < c)
    {
        m = c;
    }
    if (a == b && b == c)
    {
        m = a;
    }
    return m;
}
int min_replacements(int n, int *ds, int *ps, int *rs)
{
    int t = 0;
    if (ds[0] == ps[0] && ps[0] == rs[0] && ds[0] == rs[0])
    {
        return (n + ps[0]) * rs[0];
    }
    bool loop = true;
    while (loop)
    {
        for (int i = 0; i < n - 1; ++i)
        {
            if (ot(*(ds + i), *(ps + i), *(rs + i)) || ooo(*(ds + i), *(ps + i), *(rs + i)) || to(*(ds + i), *(ps + i), *(rs + i)))
            {
                continue;
            }
            int m = max(*(ds + i), *(ps + i), *(rs + i));
            if (m == *(ds + i))
            {
                *(ps + i + 1) += *(ps + i);
                *(rs + i + 1) += *(rs + i);
                *(ps + i) = *(rs + i) = 0;
                t += 2;
            }
            if (m == *(ps + i))
            {
                *(ds + i + 1) += *(ds + i);
                *(rs + i + 1) += *(rs + i);
                *(ds + i) = *(rs + i) = 0;
                t += 2;
            }
            if (m == *(rs + i))
            {
                *(ds + i + 1) += *(ds + i);
                *(ps + i + 1) += *(ps + i);
                *(ps + i) = *(ds + i) = 0;
                t += 2;
            }
        }
        for (int i = 0; i < n; ++i)
        {
            if (ot(*(ds + i), *(ps + i), *(rs + i)) || ooo(*(ds + i), *(ps + i), *(rs + i)) || to(*(ds + i), *(ps + i), *(rs + i)))
            {
                loop = false;
            }
            else
            {
                loop = true;
            }
        }
        if (loop)
        {
            *ds += *(ds + n - 1);
            *ps += *(ps + n - 1);
            *rs += *(rs + n - 1);
            *(ds + n - 1) = *(ps + n - 1) = *(rs + n - 1) = 0;
            t -= 2;
        }
    }
    if (t == 0)
        return 0;
    return t + 1;
}

I used a cin in this algorithm Can you help me? Thank you so much.

Answer 1

How do you know the std::cin part is the problem? Did you profile your code? If not, I suggest doing that, it's often surprising which part of the code is taking up most time. See eg How can I profile C++ code running on Linux? .

You're doing a lot of unnecessary work in various parts of the code. For example, your max function does at least 7 comparissons, and looks extremely error prone to write. You could simply replace the whole function by:

std::max({ a, b, c })

I would also take a look at your min_replacements function and see if it can be simplified. Unfortunately, you're using variable names which are super vague, so it's pretty much impossible to understand what the code should be doing. I suggest using much more descriptive variable names. That way the code will become much easier to reason about. The way it's currently written, there's a very good change even you yourself won't be able to make sense of it in a month's time.

Just glacing over the min_replacements function though, there's definitely a lot more work going on than necessary. Eg the last for-loop:

   for (int i = 0; i < n; ++i)
    {
        if (ot(*(ds + i), *(ps + i), *(rs + i)) || ooo(*(ds + i), *(ps + i), *(rs + i)) || to(*(ds + i), *(ps + i), *(rs + i)))
        {
            loop = false;
        }
        else
        {
            loop = true;
        }
    }

Each loop iterator sets the loop variable. Assuming this code is correct, you don't need the loop at all, just do the check only once for i = n - 1 . That's already O(n) changed to O(1) .