Hashing - Hard Version

Hashing - Hard Version

Given a hash table of size N, we can define a hash function . Suppose that the linear probing is used to solve collisions, we can easily obtain the status of the hash table with a given sequence of input numbers.

However, now you are asked to solve the reversed problem: reconstruct the input sequence from the given status of the hash table. Whenever there are multiple choices, the smallest number is always taken.

Input Specification:

Each input file contains one test case. For each test case, the first line contains a positive integer N (≤1000), which is the size of the hash table. The next line contains N integers, separated by a space. A negative integer represents an empty cell in the hash table. It is guaranteed that all the non-negative integers are distinct in the table.

Output Specification:
For each test case, print a line that contains the input sequence, with the numbers separated by a space. Notice that there must be no extra space at the end of each line.

Sample Input

11
33 1 13 12 34 38 27 22 32 -1 21

Sample Output:

1 13 12 21 33 34 38 27 22 32

分析

参考Messier
可以使用拓扑排序来解这道题。基本思路如下：将输入保存在散列表后，遍历每个元素，如果元素刚好在它对应余数的位置上，则入度为0，可直接输出；否则，从余数位置出发，用线性探测法到达该位置，对于经过的所有的非空元素位置，生成一条到该元素位置的边，并将该位置入度加1；拓扑排序时，可以采用优先队列，优先输出数值较小的元素

代码如下

#include<iostream>
#include<queue>
#include<algorithm> 
using namespace std;
vector<int> G[1000]; // 连接表
int N;
int indegree[1000]={0}; // 入度
int a[1000];
struct mycompare{ // 为优先队列自定义操作
    bool operator()(int m,int n){
        return a[m]>a[n];
    }
};
void toposort(){ // 拓扑排序
    int tag=0;
    priority_queue<int,vector<int>,mycompare> q;
    for(int i=0;i<N;i++)
    if(indegree[i]==0&&a[i]>=0)
    q.push(i);
    while(!q.empty()){
        int t=q.top();
        if(tag++==0) cout<<a[t];
        else cout<<" "<<a[t];
        q.pop();
        for(int i=0;i<G[t].size();i++){
            int v=G[t][i];
            indegree[v]--;
            if(indegree[v]==0)
            q.push(v);
        }
    } 
}
int main(){
    cin>>N;
    for(int i=0;i<N;i++)
    cin>>a[i];
    for(int i=0;i<N;i++){ // 建图
        if(a[i]%N==i||a[i]<0)
        continue;
        else 
        {
            int t=a[i]%N;
            while(t!=i){
                G[t].push_back(i);
                indegree[i]++;
                t=(t+1)%N;
            }
        }
    }
    toposort();
    return 0;
}