poj2376 Cleaning Shifts

题意：已知有n个区间，和一个数字t。要求用最少的区间数去覆盖[1,t]这个区间，如果不能完全覆盖，输出-1

按区间起点由大到小排序，每次都寻找覆盖的最广的区间来覆盖部分。

#include<cstdio>#include<algorithm>#define Max(a,b) a>b?a:busing namespace std;struct time{int start;int end;}data[25001];bool cmp(time a,time b){return a.start<=b.start;}int main(){int n,t,ans=0;scanf("%d%d",&n,&t);for(int i=0;i<n;i++)scanf("%d%d",&data[i].start,&data[i].end);sort(data,data+n,cmp);int index=0;int end=0;while(end<t){int begin = end+1;for(int i=index;i<n;i++){if(data[i].start<=begin){if(data[i].end>=begin)end = Max(end,data[i].end);}else{index=i;break;}}if(begin>end){printf("-1\n");return 0;}else{ans++;}}printf("%d\n",ans);return 0;}