CF 847 A. Union of Doubly Linked Lists（模拟）

Posted 2020-10-18 leonard-

题目链接：http://codeforces.com/problemset/problem/847/A

题目：

Doubly linked list is one of the fundamental data structures. A doubly linked list is a sequence of elements, each containing information about the previous and the next elements of the list. In this problem all lists have linear structure. I.e. each element except the first has exactly one previous element, each element except the last has exactly one next element. The list is not closed in a cycle.

In this problem you are given n memory cells forming one or more doubly linked lists. Each cell contains information about element from some list. Memory cells are numbered from 1 to n.

For each cell i you are given two values:

• li — cell containing previous element for the element in the cell i;
• ri — cell containing next element for the element in the cell i.

If cell i contains information about the element which has no previous element then li = 0. Similarly, if cell i contains information about the element which has no next element then ri = 0.

Three lists are shown on the picture.

For example, for the picture above the values of l and r are the following: l1 = 4, r1 = 7; l2 = 5, r2 = 0; l3 = 0, r3 = 0; l4 = 6, r4 = 1; l5 = 0, r5 = 2; l6 = 0, r6 = 4; l7 = 1, r7 = 0.

Your task is to unite all given lists in a single list, joining them to each other in any order. In particular, if the input data already contains a single list, then there is no need to perform any actions. Print the resulting list in the form of values li, ri.

Any other action, other than joining the beginning of one list to the end of another, can not be performed.

Input

The first line contains a single integer n (1 ≤ n ≤ 100) — the number of memory cells where the doubly linked lists are located.

Each of the following n lines contains two integers li, ri (0 ≤ li, ri ≤ n) — the cells of the previous and the next element of list for cell i. Value li = 0 if element in cell i has no previous element in its list. Value ri = 0 if element in cell i has no next element in its list.

It is guaranteed that the input contains the correct description of a single or more doubly linked lists. All lists have linear structure: each element of list except the first has exactly one previous element; each element of list except the last has exactly one next element. Each memory cell contains information about one element from some list, each element of each list written in one of n given cells.

Output

Print n lines, the i-th line must contain two integers li and ri — the cells of the previous and the next element of list for cell i after all lists from the input are united in a single list. If there are many solutions print any of them.

Example

input

7
4 7
5 0
0 0
6 1
0 2
0 4
1 0

output

4 7
5 6
0 5
6 1
3 2
2 4
1 0
题意：给定几个双向链表，把这几个双向链表合成一个。
题解：合链表过程中避免一下形成环。把一个节点右边是0（即没连接）和另一个左边是0的节点连接的时候判断一下他们是不是在同一只链表上面，即不断往中间找，看看最后会不会相同。
这里特别判断一下两个节点的链表，两边也是不能连起来的。

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 
 4 #define PI acos(-1.0)
 5 #define INF 0x3f3f3f3f
 6 #define FAST_IO ios::sync_with_stdio(false)
 7 #define CLR(arr,val) memset(arr,val,sizeof(arr))
 8 
 9 typedef long long LL;
10 struct TnT{
11     int l,r;
12 }T[111];
13 
14 bool check(int i,int j){
15     int t1=T[i].l,t2=T[j].r;
16     while(1){
17         if(t1==0||t2==0) return 1;
18         if((t1==j&&t2==i)||t1==t2) return 0;
19         t1=T[t1].l;t2=T[t2].r;
20     }
21 }
22 
23 int main(){
24     int n;
25     cin>>n;
26     for(int i=1;i<=n;i++){
27         cin>>T[i].l>>T[i].r;
28     }
29 
30     if(n==1){
31         cout<<T[1].l<<" "<<T[1].r<<endl;
32         return 0;
33     }
34     for(int i=1;i<=n;i++){
35         if(T[i].r==0){
36             for(int j=1;j<=n;j++){
37                 if(i==j) continue;
38                 if(T[j].l==0&&check(i,j)) {T[i].r=j;T[j].l=i;break;}
39             }
40         }
41     }
42 
43     for(int i=1;i<=n;i++){
44         cout<<T[i].l<<" "<<T[i].r<<endl;
45     }
46 
47     return 0;
48 }