POJ_2376_Cleaning Shifts【贪心】【区间覆盖】

#include <cstdio>
#include <algorithm>
using namespace std;

typedef long long ll;

const int maxn = 25000 + 5;
const int maxd = 1000000 + 5;

struct Edge
{
    int x;
    int y;
}cow[maxn];

int N, T;
int X, Y;

bool cmp(Edge a, Edge b) 
{
    if (a.x == b.x) return a.y>b.y;
    return a.x<b.x;
}

void input() 
{
    X = 0; Y = 0;
    scanf("%d%d", &N, &T);
    for (int i = 0; i<N; i++) 
    {
        scanf("%d%d", &cow[i].x, &cow[i].y);
        if (cow[i].x == 1) X = 1;
        if (cow[i].y == T) Y = 1;
    }
}

void solve() 
{
    sort(cow, cow + N, cmp);
    //若所有的牛的工作时间的起始时间和结束时间都没有覆盖掉区间的起始时间和结束时间  
    if (X == 0 || Y == 0) 
    {
        printf("-1\n");
        return;
    }
    int ans = 1, End = 0, sum = 0;
    int Start = cow[0].y;
    int maxy = cow[0].y;
    while (true) 
    {
        while (End + 1<N&&cow[End + 1].x <= Start + 1)    //End代表当前点的坐标
        {
            End++;
            if (cow[End].y>maxy) 
            {
                maxy = cow[End].y;
            }
        }
        if (maxy != Start)          //start为当前有效区间的最远距离
        {
            ans++;
            Start = maxy;
        }
        else 
        {
            if (End == N - 1)  //已覆盖掉区间
            {  
                break;
            }
            else //说明中间有的时间点覆盖不到  
            {  
                printf("-1\n");
                return;
            }
        }
    }
    printf("%d\n", ans);
    return;
}

int main() {
    input();
    solve();
    return 0;
}