思路

模拟 + 双指针

用pair类型的数组将连续天数存下来 –>
sort()排序(按first升序) –>
处理交叉的连续天数(双指针l, r) –>
用s[N]将所有的连续区间之间的相隔天数记录下来(补签卡多了) –>
模拟l -> nn(处理后的连续区间数), res接受以l为首区间的最大连续天数

#include <iostream>
#include <cstring>
#include <algorithm>

#define LL long long  //int
#define x first
#define y second

using namespace std;

const int N = 1e5 + 10;

typedef pair<LL, LL> PLL;

LL n, m;
PLL q[2 * N];
LL s[N];

int main()
{
    while(cin >> n >> m)
    {
        for(int i = 0; i < n; i ++ )
        {
            LL l, r;
            cin >> l >> r;
            q[i] = PLL(l, r);
        }
        sort(q, q + n);
        int nn = 0;
        for(int l = 0, r = l + 1; l < n; l ++ )
        {
            LL fin = q[l].y;
            while(fin >= q[r].x && r < n) 
                fin = max(fin, q[r].y), r ++ ;
            r -- ;
            q[nn ++ ] = PLL(q[l].x, fin);
            l = r;
        }
        for(int i = 1; i < nn; i ++ )
            s[i] = s[i - 1] + q[i].x - q[i - 1].y - 1;
        LL res = 0;
        for(int l = 0, r = 0; l < nn && r < nn; l ++ )
        {
            while(s[r] - s[l] <= m && r < nn) 
                r ++ ;
            r -- ;
            res = max(res, q[r].y - q[l].x + 1 + m - (s[r] - s[l]));
        }
        cout << res << endl;
    }
    return 0;
}