The first Central China Program Design Invitational competition and the tenth Wuhan University Program Design Competition Network preliminaries G questions (HDOJ-4221)

Is the question greedy? Okay .. it's really greedy .. I tried a solution a little bit .. it's over... this dislike is to sort all projects by D .. arrange work in this order .. the biggest penalty in the process is .. principle .. I just feel that the gap between the first-to-last-to be-last... no proof...

Program:

#include<stdio.h>#include<iostream>#include<string.h>#include<math.h>#define ll long longusing namespace std;struct node{      ll C,D;}a[100005];ll T,t,n,i,ans,m;bool cmp(node a,node b){      if (a.D!=b.D) return a.D<b.D;      return a.C<b.C;}int main(){      freopen("input.txt","r",stdin);      freopen("output.txt","w",stdout);      scanf("%I64d",&T);      for (t=1;t<=T;t++)      {            scanf("%I64d",&n);             for (i=1;i<=n;i++) scanf("%I64d%I64d",&a[i].C,&a[i].D);            sort(a+1,a+1+n,cmp);            ans=0;            m=0;             for (i=1;i<=n;i++)            {                   m+=a[i].C;                  if (m>a[i].D && m-a[i].D>ans) ans=m-a[i].D;            }            printf("Case %I64d: %I64d\n",t,ans);      }      return 0;}