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。选取其中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之间的数都可以得到，输出R。如果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，然后在该基础上从可以组成L的状态中挑选一个，L+1的状态中挑取一个，知道说总的挑取出所有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;
}