poj2376 贪心算法

/* poj2376 贪心算法 题目大意：用小区间覆盖大区间，输出最小的覆盖数。如果不能覆盖，则输出-1 题目分析：使用贪心算法，每次都应该让已经覆盖的区间尽可能大 测试用例：http://contest.usaco.org/DEC04_8.htm */

#include <iostream>
#include <algorithm>
#define LOCAL
#ifdef LOCAL
#include <fstream>
#endif
const int MAXT = 1000000 + 10;
const int MAXN = 25000 + 10;

typedef struct shift_
{
    int  start;
    int end;
}Shift;

//用于排序
bool cmp(Shift a, Shift b)
{
    if (a.start != b.start)
        return a.start < b.start;
    else
        return a.end < b.end;
}

int N;          //行数
int T;          //总长度
Shift a[MAXN];  //线段数组

int main()
{
#ifdef LOCAL
    FILE *s;
    if (freopen_s(&s, "poj2376.txt", "r", stdin) != 0)
    {
        std::cout << "Open file error!" << std::endl;
        return -1;
    }
#endif

    //scanf_s("%d %d", N, T);
    std::cin >> N >> T;
    for (int i = 1; i <= N; ++i)
    {
        //scanf("%d, %d", a[i].start, a[i].end);
        std::cin >> a[i].start >> a[i].end;

    }
    std::sort(a+1, a+N+1, cmp);

    int right = 1;      //已经达到的右端
    int newRight = 0;   //新的右端
    int i = 1;
    int cnt = 0;        //总段数

    while(i <= N && right <= T)
    {
        //找到一个最长的
        while (i <= N && a[i].start <= right)
        {
            newRight = std::max(newRight, a[i].end);
            ++i;
        } 

        if (newRight >= right)
        {
            cnt++;
            right = newRight+1;
        }
        else
        {
            break;
        }
    }

    if (right <= T)
    {
        printf("-1\n");
    }
    else
    {
        printf("%d\n", cnt);
    }

#ifdef LOCAL
    fclose(s);
#endif

    return 0;
}