luogu 1712

按区间长度降序排序
维护区间指针 [l, r]，第 l ~ r 条线段
表示当前区间可以满足条件
那么 r 后移一定不是更优的
因此 l 前移，使得 r 后移
过程中取最小值更新 answer

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <string>
 
using namespace std;
 
#define LL long long
 
#define gc getchar()
inline int read() {int x = ; char c = gc; while(c < '' || c > '') c = gc;
while(c >= '' && c <= '') x = x *  + c - '', c = gc; return x;}
inline LL read_LL() {LL x = ; char c = gc; while(c < '' || c > '') c = gc;
while(c >= '' && c <= '') x = x *  + c - '', c = gc; return x;}
#undef gc
 
const int N = 1e6 + ;
 
struct Node {int l, r, len;} G[N >> ];
int A[N];
int Max[N << ], W[N << ], F[N << ];
int n, m;
 
inline bool Cmp(Node a, Node b) {return a.len > b.len;}
 
#define lson jd << 1
#define rson jd << 1 | 1
 
void Push_down(int jd) {
    F[lson] += F[jd], F[rson] += F[jd];
    Max[lson] += F[jd], Max[rson] += F[jd];
    F[jd] = ;
}
 
void Sec_G(int l, int r, int jd, int x, int y, int num) {
    if(x <= l && r <= y) {
        Max[jd] += num;
        F[jd] += num;
        return ;
    }
    if(F[jd]) Push_down(jd);
    int mid = (l + r) >> ;
    if(x <= mid) Sec_G(l, mid, lson, x, y, num);
    if(y > mid ) Sec_G(mid + , r, rson, x, y, num);
    Max[jd] = max(Max[lson], Max[rson]);
}
 
bool use[N];
 
int main() {
    n = read(), m = read();
    int tot = ;
    for(int i = ; i <= n; i ++) {
        G[i].l = read(), G[i].r = read(), G[i].len = G[i].r - G[i].l; A[++ tot] = G[i].l, A[++ tot] = G[i].r;
    }
    sort(G + , G + n + , Cmp);
    sort(A + , A + tot + );
    for(int i = ; i <= n; i ++) {
        G[i].l = lower_bound(A + , A + tot + , G[i].l) - A;
        G[i].r = lower_bound(A + , A + tot + , G[i].r) - A;
    }
    int R = , Answer = (int)1e9 + ;
    for(int i = ; i <= n; i ++) {
        if(R == n) break;
        if(use[i] == ) {
            Sec_G(, tot, , G[i].l, G[i].r, );
            use[i] = ;
        }
        while(Max[] < m && R < n) {
            R ++;
            Sec_G(, tot, , G[R].l, G[R].r, );
            use[R] = ;
            if(Max[] >= m) {
                Answer = min(Answer, G[i].len - G[R].len);
                break;
            }
        }
        if(use[i]) {
            Sec_G(, tot, , G[i].l, G[i].r, -);
            use[i] = ;
        }
    }
    if(Answer == (int)1e9 + ) cout << "-1";
    else cout << Answer;
    return ;
}