usaco training 6.1 牛異或 字首異或和+異或字典樹差不多的題目，只不過增加了異或字典樹的刪除。

#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 100010 * 31, M = 100010;

int n, m;
int s[M];
int son[N][2], cnt[N], idx;

void insert(int x, int v)
{
    int p = 0;
    for (int i = 30; i >= 0; i -- )
    {
        int u = x >> i & 1;
        if (!son[p][u]) son[p][u] = ++ idx;
        p = son[p][u];
        cnt[p] += v;
    }
}

int query(int x)
{
    int res = 0, p = 0;
    for (int i = 30; i >= 0; i -- )
    {
        int u = x >> i & 1;
        if (cnt[son[p][!u]]) p = son[p][!u], res = res * 2 + 1;
        else p = son[p][u], res = res * 2;
    }
    return res;
}

int main()
{
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; i ++ )
    {
        int x;
        scanf("%d", &x);
        s[i] = s[i - 1] ^ x;
    }

    int res = 0;
    insert(s[0], 1);

    for (int i = 1; i <= n; i ++ )
    {
        if (i > m) insert(s[i - m - 1], -1);
        res = max(res, query(s[i]));
        insert(s[i], 1);
    }

    printf("%d\n", res);
    return 0;
}