一、题目大意

就是求中间的数。

二、AC code

递归快排ac版：

#include <iostream>
#include <stdio.h>
#include <cstring>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <cassert>
#include <time.h>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <string>
#include <sstream>
#include <list>
#define INF 0x3f3f3f3fusing namespace std;template <class Type>
Type stringToNum(const string& str)
{istringstream iss(str);Type num;iss >> num;return num;
}//======================================================#define MAXN 250002int a[MAXN];void quickSort(int *arr, int left, int right){int i = left, j = right;int mid = arr[(i+j)/2];while(i <= j){while(arr[i] < mid) i ++;while(arr[j] > mid) j --;if(i <= j){int tmp;tmp = arr[i]; arr[i] = arr[j]; arr[j] = tmp;i ++; j --;}}if(i < right) quickSort(arr,i, right);if(left < j) quickSort(arr,left, j);
}int main()
{//freopen("input.txt","r",stdin);int N;cin>>N;for (int i = 1; i <= N; ++i) {scanf("%d",&a[i]);}quickSort(a,1,N);if(N%2) //oddprintf("%d.0\n", a[(N+1)/2]);else {double sum = (double)a[N/2] + (double)a[N/2+1];printf("%.1lf\n", sum / 2);}return 0;
}

可以水过，但是不知道为什么用g++也会WA，用c++ OK
同时因为数的范围过大，用计数排序不靠谱。

#include <iostream>
#include <stdio.h>
#include <cstring>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <cassert>
#include <time.h>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <string>
#include <sstream>
#define INF 0x3f3f3f3fusing namespace std;template <class Type>
Type stringToNum(const string& str)
{istringstream iss(str);Type num;iss >> num;return num;
}//======================================================#define MAXN 250002long long a[MAXN];int main()
{//freopen("input.txt","r",stdin);int N;cin>>N;for (int i = 1; i <= N; ++i) {scanf("%lld",&a[i]);}sort(a+1,a+N+1);//oddif(N%2)printf("%lld.0\n", a[(N+1)/2]);else {//printf("%.1lf\n", ((double)a[N/2]+(double)a[(N/2)+1])/2); //ok//ok
//      double sum = (double)a[N/2]+(double)a[(N/2)+1];
//      printf("%.1lf\n",sum / 2);printf("%.1lf\n", (double)(a[N/2]+a[(N/2)+1])/2);}return 0;
}

stl二分会超时：

#include <iostream>
#include <stdio.h>
#include <cstring>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <cassert>
#include <time.h>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <string>
#include <sstream>
#include <list>
#define INF 0x3f3f3f3fusing namespace std;template <class Type>
Type stringToNum(const string& str)
{istringstream iss(str);Type num;iss >> num;return num;
}//======================================================#define MAXN 250002vector<int > v;//有序数组递减排列
int  binarySearch(vector<int > array,int len,int value){int mid=0;int low=0;int high=len-1;while(low<=high){mid=(low+high)/2;if(array[mid]>value){       //在右半区low=mid+1;continue;}else if(array[mid]<value){  //在左半区high=mid-1;continue;}elsereturn mid;             //找到}return low;  //insert pos
}int main()
{//freopen("input.txt","r",stdin);int N;cin>>N;for (int i = 1; i <= N; ++i) {long long tmp;scanf("%lld",&tmp);int pos = binarySearch(v,i-1,tmp);v.insert(v.begin() + pos, tmp);}if(N%2) //oddprintf("%lld.0\n", v[(N+1)/2-1]);elseprintf("%.1lf\n", (double)(v[N/2-1]+v[N/2])/2);return 0;
}