主席树 - 查询某区间第 K 大

#define ll long long
const int maxn = 1e5+5;
const int mod = 1e9+7;
const double eps = 1e-9;
const double pi = acos(-1.0);
const int inf = 0x3f3f3f3f;

int n, m;
int pre[maxn];
int rank[maxn];
int cnt, ss;
int root[maxn];
struct node
{
    int l, r;
    int sum;
}t[maxn*20];

void init(){
    cnt = 1;
    root[0] = 0;
    t[0].l = t[0].r = t[0].sum = 0;        
}

void update(int num, int &rt, int l, int r){
    t[cnt++] = t[rt];
    rt = cnt-1;
    t[rt].sum++;
    if (l == r) return;
    int m = (l+r)>>1;
    if (num <= m) update(num, t[rt].l, l, m);
    else update(num, t[rt].r, m+1, r);
}

int query(int i, int j, int k, int l, int r){
    int d = t[t[j].l].sum - t[t[i].l].sum;
    int m = (l+r)>>1;
    
    if (l == r) return l;
    if (k <= d) return query(t[i].l, t[j].l, k, l, m);
    else return query(t[i].r, t[j].r, k-d, m+1, r);
}

int main() {
    //freopen("in.txt", "r", stdin);
    //freopen("out.txt", "w", stdout);
    cin >> n >> m;
    for(int i = 1; i <= n; i++){
        scanf("%d", &pre[i]);
        rank[i] = pre[i];
    }
    sort(rank+1, rank+1+n);
    ss = unique(rank+1, rank+1+n)-rank;
    //printf("--  %d\n", cnt);
    init();
    for(int i = 1; i <= n; i++){
        int x = lower_bound(rank+1, rank+ss, pre[i])-rank;
        root[i] = root[i-1];
        update(x, root[i], 1, n);
    }
    int a, b, c;
    for(int i = 1; i <= m; i++){
        scanf("%d%d%d", &a, &b, &c);
        int ans = query(root[a-1], root[b], c, 1, n);
        printf("%d\n", rank[ans]);
    }
    return 0;
}