Codeforces 1175E Minimal Segment Cover 倍增

Minimal Segment Cover

嗯嗯嗯。。 打倍增的时候两个for写反了查了半天bug。。。

我们先把没用的线段去掉， 然后对每个线段都找到和它接触的r最大的作为下一个， 然后打倍增就好了。

#include<bits/stdc++.h>
#define LL long long
#define LD long double
#define ull unsigned long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ALL(x) (x).begin(), (x).end()
#define fio ios::sync_with_stdio(false); cin.tie(0);

using namespace std;

const int N = 5e5 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 998244353;
const double eps = 1e-8;
const double PI = acos(-1);

template<class T, class S> inline void add(T& a, S b) {a += b; if(a >= mod) a -= mod;}
template<class T, class S> inline void sub(T& a, S b) {a -= b; if(a < 0) a += mod;}
template<class T, class S> inline bool chkmax(T& a, S b) {return a < b ? a = b, true : false;}
template<class T, class S> inline bool chkmin(T& a, S b) {return a > b ? a = b, true : false;}

int n, m, c[N];
int prefix[N];
int nex[N][20];

struct Line {
    int l, r;
    bool operator < (const Line &rhs) const {
        if(l == rhs.l) return r > rhs.r;
        return l < rhs.l;
    }
} a[N];

int main() {
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= n; i++) scanf("%d%d", &a[i].l, &a[i].r), a[i].r--;
    sort(a + 1, a + 1 + n);
    int nn = n; n = 1;
    for(int i = 2; i <= nn; i++)
        if(a[i].r > a[n].r) a[++n] = a[i];
    for(int i = 1; i <= n; i++) {
        c[a[i].l]++;
        c[a[i].r + 1]--;
    }
    if(!c[0]) prefix[0] = 1;
    for(int i = 1; i <= 500000; i++) {
        c[i] += c[i - 1];
        if(!c[i]) prefix[i] = 1;
        prefix[i] += prefix[i - 1];
    }
    for(int i = 1, j = 2; i <= n; i++) {
        j = max(i + 1, j);
        while(j <= n && a[j].l <= a[i].r + 1) j++;
        nex[i][0] = j - 1 == i ? 0 : j - 1;
    }
    for(int j = 1; j < 20; j++)
        for(int i = 1; i <= n; i++)
            nex[i][j] = nex[nex[i][j - 1]][j - 1];
    while(m--) {
        int x, y; scanf("%d%d", &x, &y);
        y--;
        int cnt = prefix[y];
        if(x) cnt -= prefix[x - 1];
        if(cnt) {
            puts("-1");
        } else {
            int p = lower_bound(a + 1, a + 1 + n, Line{x + 1, inf}) - a - 1;
            if(a[p].r >= y) {
                puts("1");
            } else {
                int ans = 0;
                for(int i = 19; i >= 0; i--) {
                    if(nex[p][i] && a[nex[p][i]].r < y) {
                        p = nex[p][i], ans += (1 << i);
                    }
                }
                printf("%d\n", ans + 2);
            }
        }
    }
    return 0;
}