题目链接：hdu 4876 ZCC loves cards

题目大意：给出n，k，l，表示有n张牌，每张牌有值。选取当中k张排列成圈，然后在该圈上进行游戏，每次选取m(1≤m≤k)张连续的牌，取牌上值的亦或和。要求找到一个圈，使得L~R之间的数都能够得到，输出R。假设R < L输出0.

解题思路：暴力，首先预处理出来每种选取的亦或值，然后在该基础上从能够组成L的状态中挑选一个，L+1的状态中挑取一个，知道说总的挑取出全部状态中选中的牌的个数大于K为值，然后用全排序去查找最大的R。

#include <cstdio>

#include <cstring>

#include <vector>

#include <map>

#include <algorithm>

using namespace std;

typedef long long ll;

const int maxn = 200;

const int maxc = 25;

const int maxs = 1<<20;

const int maxt = 20005;

bool flag;

int ans, rec, dp[maxs+5];

int N, K, L, arr[maxc];

int vec[maxn][maxt], c[maxn];

inline int bitcount (int s) {

    return s == 0 ? 0 : bitcount(s/2) + (s&1);

}

void dfs (int d, int s, int val, int f) {

    if (d == K + 1)

        return;

    vec[val][c[val]++] = s;

    for (int i = f + 1; i < N; i++)

        dfs (d+1, s|(1<<i), val^arr[i], i);

}

void init () {

    flag = true;

    ans = rec = 0;

    memset(c, 0, sizeof(c));

    memset(dp, 0, sizeof(dp));

    for (int i = 0; i < N; i++)

        scanf("%d", &arr[i]);

    dfs(0, 0, 0, -1);

}

int check (int* a) {

    int v[maxn];

    memset(v, 0, sizeof(v));

    for (int m = 1; m <= K; m++) {

        int tmp = a[0];

        for (int i = 1; i < m; i++)

            tmp ^= a[i];

        for (int i = 0; i < K; i++) {

            v[tmp] = 1;

            tmp ^= (a[i] ^ a[i+m]);

        }

    }

    int ans = L;

    while (v[ans] && ans < maxn)

        ans++;

    return ans - 1;

}

void judge (int s) {

    int cnt = 0, a[2*maxn];

    for (int i = 0; i < N; i++) {

        if (s&(1<<i))

            a[cnt++] = arr[i];

    }

    sort(a + 1, a + cnt);

    do {

        for (int i = 0; i < cnt; i++)

            a[i+cnt] = a[i];

        ans = max(ans, check(a));

    } while (next_permutation(a + 1, a + cnt));

}

/*

*/

void solve (int d, int s) {

    int cnt = bitcount(s);

    rec = max(d, rec);

    if (cnt >= K || d >= maxn) {

        if (cnt == K) {

            judge(s);

            flag = false;

        }

        return;

    }

    for (int i = 0; i < c[d]; i++) {

        int s0 = (s | vec[d][i]);

        if (dp[s0]) continue;

        solve(d+1, s0);

        dp[s0] = 1;

    }

}

int main () {

    while (scanf("%d%d%d", &N, &K, &L) == 3) {

        init();

        solve(L, 0);

        if (flag)

            ans = rec - 1;

        printf("%d\n", ans < L ? 0 : ans);

    }

    return 0;

}