hduoj4883！【数学】

/*TIANKENG’s restaurantTime Limit: 2000/1000 MS (Java/Others)    Memory Limit: 131072/65536 K (Java/Others)Total Submission(s): 675    Accepted Submission(s): 320Problem DescriptionTIANKENG manages a restaurant after graduating from ZCMU, and tens of thousands of customers come to have meal because of its delicious dishes. Today n groups of customers come to enjoy their meal, and there are Xi persons in the ith group in sum. Assuming that each customer can own only one chair. Now we know the arriving time STi and departure time EDi of each group. Could you help TIANKENG calculate the minimum chairs he needs to prepare so that every customer can take a seat when arriving the restaurant? InputThe first line contains a positive integer T(T<=100), standing for T test cases in all.Each cases has a positive integer n(1<=n<=10000), which means n groups of customer. Then following n lines, each line there is a positive integer Xi(1<=Xi<=100), referring to the sum of the number of the ith group people, and the arriving time STi and departure time Edi(the time format is hh:mm, 0<=hh<24, 0<=mm<60), Given that the arriving time must be earlier than the departure time.Pay attention that when a group of people arrive at the restaurant as soon as a group of people leaves from the restaurant, then the arriving group can be arranged to take their seats if the seats are enough.OutputFor each test case, output the minimum number of chair that TIANKENG needs to prepare.Sample Input226 08:00 09:005 08:59 09:5926 08:00 09:005 09:00 10:00Sample Output116 SourceBestCoder Round #2 */#include<stdio.h>#include<string.h>int a[1441];int main(){int num, n, m , i, j, k, e, f;scanf("%d", &n);while(n--){memset(a, 0, sizeof(a));scanf("%d", &m);for(i = 0; i  < m; i++){scanf("%d %d:%d %d:%d", &num, &j, &k, &e, &f);a[j * 60 + k] += num;a[e * 60 + f] -= num;}int sum = 0,ans = 0;for(i = 0; i  < 1441; i++){sum += a[i];if(sum > ans)ans = sum;}printf("%d\n", ans);}return 0;}

区间选点

先将基本单位化作分钟，再开个一天1440分钟的数组，保存每个点的客人数，再进行求和，求出过程中的最大值。

关键：理解为什么区间上求和取最大值就是结果。