怎么一直超时啊

[code]#include<iostream>
#include <algorithm>
#include <vector>
using namespace std;
struct str
{ int z1,z2;
};
bool compare(str m1,str m2)
     { return m1.z2<m2.z2;
     };
int main()
{int n,s,a,b,total=0,i=0;
 cin>>n>>s>>a>>b;
 int c=a+b,nn=n;
 vector<str> trees;
 str tree;
 while(nn--)
     {cin>>tree.z1>>tree.z2;
     trees.push_back(tree);
     };
 sort(trees.begin(),trees.end(),compare);
 while(s>0&&i<n)
     {if(trees[i].z1<=c&&trees[i].z2<=s)
     {
     s=s-trees[i].z2;
     ++total;
     ++i;}
     else if(trees[i].z2-s>0) break;
     };
     cout<<total<<endl;
     return 0;    
}[/code]我的代码与题解1楼相比似乎就是复杂了一点，但在循环条件判定上我是有优势的