https://www.luogu.com.cn/discuss/602529

#include<bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define inf 0x3f3f3f3f3f3f3f3f
#define clz __builtin_clz
#define lg(x) (31 - clz(x))
using namespace std;
const int maxn = 5e5+10,lim = lg(maxn)+1;
int n,k;
struct node{ int s[2],siz; }t[maxn];
int cnt,rt[maxn],a[maxn];
inline void insert(int *u,int x) {
    for(int i = lim-1;~i;--i) {
        t[++cnt] = t[*u], *u = cnt;
        ++t[*u].siz;
        const int c = x >> i & 1;
        u = t[*u].s+c;
    }
    t[++cnt] = t[*u], *u = cnt;
    ++t[*u].siz;
}
int get(int u,int x,int k) {
    int ans = 0;
    for(int i = lim-1;~i;--i) {
        ans <<= 1;
        const int c = x >> i & 1,less = t[u].s[!c][t].siz;
        if(less >= k) u = t[u].s[!c],ans |= 1;
        else k -= less,u = t[u].s[c];
    } return ans;
}
struct val {
    int p,v,k;
    val(int p,int k):p(p),k(k) { v = get(rt[p],a[p],k); }
    inline bool operator<(const val& t) const { return v < t.v; }
};
priority_queue<val> pq;
signed main() {
    ios::sync_with_stdio(0),cin.tie(0);
    cin>>n>>k;
    insert(rt,0);
    for(int i = 1;i<=n;++i) cin>>a[i],insert(&(rt[i] = rt[i-1]),a[i] ^= a[i-1]);
    for(int i = 1;i<=n;++i) {
        val t(i,1);
        pq.push(t);
    }
    int ans = 0;
    while(k--) {
        const val t = pq.top(),p(t.p,t.k+1);
        pq.pop();
        ans += t.v;
        pq.push(p);
    }
    cout<<ans;
}

调不出来了，如何理解 k乘2 这句话？