1101. Quick Sort

1101. Quick Sort (25)

时间限制

200 ms

内存限制

65536 kB

代码长度限制

16000 B

判题程序

Standard

作者

CAO, Peng

There is a classical process named partition in the famous quick sort algorithm. In this process we typically choose one element as the pivot. Then the elements less than the pivot are moved to its left and those larger than the pivot to its right. Given N distinct positive integers after a run of partition, could you tell how many elements could be the selected pivot for this partition?

For example, given N = 5 and the numbers 1, 3, 2, 4, and 5. We have:

1 could be the pivot since there is no element to its left and all the elements to its right are larger than it;

3 must not be the pivot since although all the elements to its left are smaller, the number 2 to its right is less than it as well;

2 must not be the pivot since although all the elements to its right are larger, the number 3 to its left is larger than it as well;

and for the similar reason, 4 and 5 could also be the pivot.

Hence in total there are 3 pivot candidates.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (<= 10⁵). Then the next line contains N distinct positive integers no larger than 10⁹. The numbers in a line are separated by spaces.

Output Specification:

For each test case, output in the first line the number of pivot candidates. Then in the next line print these candidates in increasing order. There must be exactly 1 space between two adjacent numbers, and no extra space at the end of each line.

Sample Input:

51 3 2 4 5

Sample Output:

31 4 5

第一遍输入，把满足左边数小于该数的数标记，第二次从后往前遍历，标记第一次被标记而且所有右边数大于该数的数。

输出即可，不用排序，因为肯定满足条件，最后记得输出一个换行。

#include<iostream>#include<vector>#include<set>using namespace std;int main(){int n;int cnt=0;cin>>n;vector<int>num(n);vector<bool>pre(n);vector<bool>ans(n);int maxn=0;for(int i=0;i<n;++i){cin>>num[i];if(num[i]>maxn){maxn=num[i];pre[i]=true;}elsepre[i]=false;}maxn=0x7fffffff;for(int i=n-1;i>=0;--i){if(num[i]<maxn){maxn=num[i];if(pre[i]){ans[i]=true;cnt++;continue;}}ans[i]=false;}cout<<cnt<<endl;bool f=false;for(int i=0;i<n;++i)if(ans[i]&&f)cout<<" "<<num[i];else if(ans[i]&&!f){cout<<num[i];f=true;}cout<<endl;return 0;}