poj2456 Aggressive cows（二分查找）

https://vjudge.net/problem/POJ-2456

二分，从最大长度开始，不断折半试，如果牛全放下了，就是可行，修改下界，否则改上届。

 #include<iostream>
 #include<cstdio>
 #include<queue>
 #include<cstring>
 #include<algorithm>
 #include<cmath>
 #include<map>
 #define lson l, m, rt<<1
 #define rson m+1, r, rt<<1|1
 #define INF 0x3f3f3f3f
 typedef unsigned long long ll;
 using namespace std;
 int n, m, a[];
 int C(int x)
 {
     int pre=, ans=;
     for(int i = ; i < n; i++){
         while(i < n&&a[i]-a[pre]<x){
             i++;
         }
         if(i>=n) break;
         ans++;
         pre = i;
     }
     return ans >= m;
 }
 int main()
 {
     while(~scanf("%d%d", &n, &m)){
         for(int i = ; i < n; i++){
             scanf("%d", &a[i]);
         }
         sort(a, a+n);
         int lb = , ub = INF;
         while(ub-lb>){
             int mid = (ub+lb)/;
             if(C(mid)){
                 lb = mid;
             }
             else ub = mid;
         }
         printf("%d\n", lb);
     }
     return ;
 }