【39.77%】【codeforces 724B】Batch Sort

time limit per test2 seconds
memory limit per test256 megabytes
inputstandard input
outputstandard output
You are given a table consisting of n rows and m columns.

Numbers in each row form a permutation of integers from 1 to m.

You are allowed to pick two elements in one row and swap them, but no more than once for each row. Also, no more than once you are allowed to pick two columns and swap them. Thus, you are allowed to perform from 0 to n + 1 actions in total. Operations can be performed in any order.

You have to check whether it’s possible to obtain the identity permutation 1, 2, …, m in each row. In other words, check if one can perform some of the operation following the given rules and make each row sorted in increasing order.

Input
The first line of the input contains two integers n and m (1 ≤ n, m ≤ 20) — the number of rows and the number of columns in the given table.

Each of next n lines contains m integers — elements of the table. It’s guaranteed that numbers in each line form a permutation of integers from 1 to m.

Output
If there is a way to obtain the identity permutation in each row by following the given rules, print “YES” (without quotes) in the only line of the output. Otherwise, print “NO” (without quotes).

Examples
input
2 4
1 3 2 4
1 3 4 2
output
YES
input
4 4
1 2 3 4
2 3 4 1
3 4 1 2
4 1 2 3
output
NO
input
3 6
2 1 3 4 5 6
1 2 4 3 5 6
1 2 3 4 6 5
output
YES
Note
In the first sample, one can act in the following way:

Swap second and third columns. Now the table is
1 2 3 4
1 4 3 2
In the second row, swap the second and the fourth elements. Now the table is
1 2 3 4
1 2 3 4

【题解】

先枚举那个交换整列的操作。
然后用这道题的做法判断每一行是不是可以在每一行交换一次元素过后变成有序:
http://blog.csdn.net/harlow_cheng/article/details/52294871
就是找循环节吧。交换

#include <cstdio>
#include <algorithm>
#include <iostream>

using namespace std;

const int MAXN = 25;

struct abc
{
    int data;
    int key;
};

int n, m;
int a[MAXN][MAXN];
abc b[MAXN];
bool vis[MAXN];

void input(int &r)
{
    r = 0;
    char t = getchar();
    while (!isdigit(t)) t = getchar();
    while (isdigit(t)) r = r * 10 +t- '0', t = getchar();
}

bool cmp(abc a, abc b)
{
    return a.data < b.data;
}

int get_num(int x)
{
    for (int i = 1; i <= m; i++)
        b[i].data = a[x][i], b[i].key = i;
    sort(b + 1, b + 1 + m, cmp);
    for (int i = 1; i <= m; i++)
        vis[i] = false;
    int num = 0;
    for (int i = 1; i <= m; i++)
        if (!vis[i])
        {
            int t = i;
            int len = 0;
            while (!vis[t])
            {
                vis[t] = true;
                len++;
                t = b[t].key;
            }
            num += len - 1;//len-1就是需要交换的次数
        }
    return num;
}

bool check()
{
    for (int i = 1; i <= n; i++)
    {
        int num = get_num(i);
        if (num > 1)
            return false;
    }
    return true;
}

int main()
{
    //freopen("F:\\rush.txt", "r", stdin);
    input(n); input(m);
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            input(a[i][j]);
    bool flag = false;
    flag = check();
    if (flag)
    {
        puts("YES");
        return 0;
    }
    for (int i = 1;i <= m-1;i++)
        for (int j = i + 1; j <= m; j++)
        {
            for (int k = 1; k <= n; k++)
                swap(a[k][i], a[k][j]);
            flag = check();
            if (flag)
            {
                puts("YES");
                return 0;
            }
            for (int k = 1; k <= n; k++)
                swap(a[k][i], a[k][j]);
        }
    puts("NO");
    return 0;
}