CF1712D

wa了快一个月，原来是维护区间最值的树状数组的更新函数写挂了，:)

这错误写法能过洛谷完全因为它是不带修改的…

正确的：

struct BIT
{
    LL  h[(int)1e5 + 9], n;
    void init(int x)
    {
        n = x;
        f(i,1,n)update(i);
    }
    void update(int x)
    {
        while(x<=n){
            h[x]=aa[x];
            int low=lowbit(x);
            for(int i=1;i<low;i<<=1)h[x]=min(h[x],h[x-i]);
            x+=lowbit(x);
        }  
    }
    LL gmin(int l, int r)
    {
        LL ans = 1e18;
        while (r >= l)
        {
            ans = min(ans, aa[r]);
            r--;
            while (r - lowbit(r) >= l)
            {
                ans = min(h[r], ans);
                r -= lowbit(r);
            }
        }
        return ans;
    }
};

错误的：

struct BIT
{
    LL  h[(int)1e5 + 9], n;
    void init(int x)
    {
        n = x;
        f(i,1,n)update(i);
    }
    void update(int x,int k)
    {
        while(x<=n){
            h[x]=k;//错在这里！！！！
            int low=lowbit(x);
            for(int i=1;i<low;i<<=1)h[x]=min(h[x],h[x-i]);
            x+=lowbit(x);
        }  
    }
    LL gmin(int l, int r)
    {
        LL ans = 1e18;
        while (r >= l)
        {
            ans = min(ans, aa[r]);
            r--;
            while (r - lowbit(r) >= l)
            {
                ans = min(h[r], ans);
                r -= lowbit(r);
            }
        }
        return ans;
    }
};

#include <bits/stdc++.h>
/*
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/hash_policy.hpp>
*/
using namespace std;

const double eps = 1e-10;
const double pi = 3.1415926535897932384626433832795;
const double eln = 2.718281828459045235360287471352;

#define f(i, a, b) for (int i = a; i <= b; i++)
#define LL long long
#define IN freopen("in.txt", "r", stdin)
#define OUT freopen("out.txt", "w", stdout)
#define scan(x) scanf("%d", &x)
#define mp make_pair
#define pb push_back
#define sqr(x) (x) * (x)
#define pr1(x) printf("Case %d: ",x)
#define pn1(x) printf("Case %d:\n",x)
#define pr2(x) printf("Case #%d: ",x)
#define pn2(x) printf("Case #%d:\n",x)
#define lowbit(x) (x&(-x))

#define fi first
#define se second
#define sz(x) int((x).size())
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define summ(a) (accumulate(all(a), 0ll))

typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
using ll=long long;

LL tt,k,n,aa[(int)1e5+9],rk[(int)1e5+9];

struct BIT
{
    LL  h[(int)1e5 + 9], n;
    void init(int x)
    {
        n = x;
        f(i,1,n)update(i);
    }
    void update(int x)
    {
        while(x<=n){
            h[x]=aa[x];
            int low=lowbit(x);
            for(int i=1;i<low;i<<=1)h[x]=min(h[x],h[x-i]);
            x+=lowbit(x);
        }  
    }
    LL gmin(int l, int r)
    {
        LL ans = 1e18;
        while (r >= l)
        {
            ans = min(ans, aa[r]);
            r--;
            while (r - lowbit(r) >= l)
            {
                ans = min(h[r], ans);
                r -= lowbit(r);
            }
        }
        return ans;
    }
};


void solve(){
    cin>>n>>k;
    f(i,1,n)cin>>aa[i],rk[i]=i;
    ll ans=0;
    BIT b;
    if(k==0){
        b.init(n);
        for(int i=1;i<n;++i){
            ans=max(ans,min(min(aa[i],aa[i+1]),b.gmin(1,n)*2ll));
        }
    }
    else {
        sort(rk+1,rk+1+n,[](long long x,long long y){
            return aa[x]<aa[y];
        });
        for(int i=1;i<k;++i)aa[rk[i]]=1e9;
        b.init(n);
        aa[n+1]=-10086;
        int ma=0,ma2=0;
        for(int i=1;i<n;++i){
            int tp=min(aa[i],aa[i+1]);
            if(tp>=ma){
                ma2=ma,ma=tp;
            }else if(tp>ma2)ma2=tp;
        }
        for(int i=1;i<=n;++i){
            LL tmp=aa[i];
            aa[i]=1e9;
            b.update(i);
            int usma=max(min(aa[i],aa[i+1]),min(aa[i-1],aa[i]))==ma&&aa[i]==ma?ma2:ma;
            usma=max({aa[i-1],aa[i+1],usma+0ll});
            ans=max(ans,min(usma+0ll,b.gmin(1,n)*2));
            //cerr<<i<<"::"<<usma<<"__"<<b.gmin(1,n)<<"+++"<<ans<<"\n";
            aa[i]=tmp;
            b.update(i);
        }
    }
    cout<<ans<<"\n";
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin>>tt;
    f(sb,1,tt){
        solve();
    }
    return 0;
}