Codeforces 1003E Tree Constructing

 1 #include<iostream>
 2 #include<vector>
 3 #include<map>
 4 #include<queue>
 5 using namespace std;
 6 
 7 int degree[400005],count,pool,k; 
 8 vector<pair<int,int> > ans;
 9 
10 int top;//top是上限层数 
11 
12 void bfs(int i){//在i的后面加子树
13     //如果depth=0，那加k-2个，其他加k-1个
14     queue<pair<int,int> > q;
15     q.push( make_pair(i,0) );
16     while(!q.empty()){
17         if(count==0) break;
18         
19         pair<int,int> d = q.front(); q.pop();
20         int index = d.first,depth=d.second;
21         if(depth==top) continue;
22         if(depth==0){
23             if(count<=k-2){
24                 for(int i=1;i<=count;i++) {
25                     ans.push_back( make_pair(index,++pool) );
26                     q.push( make_pair(pool,depth+1) );
27                 }
28                 count=0;
29             }
30             else{
31                 for(int i=1;i<=k-2;i++) {
32                     ans.push_back( make_pair(index,++pool) );
33                     q.push( make_pair(pool,depth+1) );
34                 }
35                 count-=k-2;
36             }
37         }
38         else{
39             if(count<=k-1){
40                 for(int i=1;i<=count;i++) {
41                     ans.push_back( make_pair(index,++pool) );
42                     q.push( make_pair(pool,depth+1) );
43                 }
44                 count=0;
45             }
46             else{
47                 for(int i=1;i<=k-1;i++) {
48                     ans.push_back( make_pair(index,++pool) );
49                     q.push( make_pair(pool,depth+1) );
50                 }
51                 count-=k-1;
52             }
53         }
54     
55     }
56 }
57 
58 int main(){
59     int n,d; cin>>n>>d>>k;
60     
61     if(d>=n) { cout<<"NO"; return 0; }
62     for(int i=1;i<=d;i++){//用d+1个点建长度为d的链
63         degree[i]++; degree[i+1]++;
64         if(degree[i]>k || degree[i+1]>k) { cout<<"NO"; return 0; }
65         ans.push_back( make_pair(i,i+1) );
66     }
67     
68     //第二个到第d个节点，每个后面加一棵深度为min(i-1,d-i+1)的树
69     count=n-d-1;//还剩n-d-1个点要放 
70     pool=d+1;//加的点编号从d+2开始用 
71     
72     for(int i=2;i<=d;i++){
73         top=min(i-1,d-i+1);
74         bfs(i);
75         if(count==0) break; 
76     }
77     
78     if(count>0) { cout<<"NO"; return 0; }
79     cout<<"YES"<<endl;
80     for(vector<pair<int,int> >::iterator it = ans.begin(); it!=ans.end(); it++){
81         printf("%d %d\n",it->first,it->second);
82     }
83     
84     return 0;
85 }