排序+贪心
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#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <algorithm>
using namespace std;
#define maxn 25005
struct Interval
{
int s, e;
}interval[maxn];
int n, t;
bool operator < (const Interval &a, const Interval &b)
{
if (a.s != b.s)
return a.s < b.s;
return a.e < b.e;
}
int main()
{
//freopen("t.txt", "r", stdin);
scanf("%d%d", &n, &t);
for (int i = 0; i < n; i++)
scanf("%d%d", &interval[i].s, &interval[i].e);
sort(interval, interval + n);
int temp = 0;
int ans = 0;
int i = 0;
while (temp < t && i < n)
{
ans++;
int top = temp;
if (interval[i].s > temp + 1)
{
printf("-1\n");
return 0;
}
while (i < n && interval[i].s <= temp + 1)
{
top = max(top, interval[i].e);
i++;
}
temp = top;
}
if (temp < t)
printf("-1\n");
else
printf("%d\n", ans);
return 0;
}