codeforces C. Restore Graph

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <algorithm>
 4 #include <iostream>
 5 #include <cmath>
 6 #include <vector>
 7 #define maxn 100010
 8 using namespace std;
 9 
10 int n,k;
11 int d[maxn];
12 int du[maxn];
13 struct node
14 {
15     int x,id;
16     bool operator <(const node &a)const
17     {
18         return x<a.x;
19     }
20 } p[maxn];
21 vector<int>g[maxn];
22 
23 void dfs(int x)
24 {
25     for(int i=0; i<(int)g[x].size(); i++)
26     {
27         printf("%d %d\n",x,g[x][i]);
28         dfs(g[x][i]);
29     }
30 }
31 
32 int main()
33 {
34     scanf("%d%d",&n,&k);
35     for(int i=1; i<=n; i++)
36     {
37         scanf("%d",&d[i]);
38         p[i].x=d[i];
39         p[i].id=i;
40     }
41     sort(p+1,p+n+1);
42     if(p[1].x==0&&p[2].x==0||p[1].x!=0)
43     {
44         printf("-1\n");
45         return 0;
46     }
47     int l=1,r=2;
48     int cnt=0;
49     bool flag=false;
50     while(r<=n)
51     {
52         if(p[r].x==p[l].x)
53         {
54             printf("-1\n");
55             return 0;
56         }
57         if(p[r].x-p[l].x==1&&du[p[l].id]<k&&du[p[r].id]<k)
58         {
59             g[p[l].id].push_back(p[r].id);
60             cnt++;
61             du[p[l].id]++;
62             du[p[r].id]++;
63             r++;
64         }
65         else if(p[r].x-p[l].x>1||du[p[l].id]>=k)
66         {
67             l++;
68             if(l==r)
69             {
70                 flag=true;
71                 break;
72             }
73         }
74         else if(du[p[r].id]>=k)
75         {
76             r++;
77         }
78         else
79         {
80             r++;
81         }
82     }
83     if(flag)
84     {
85         printf("-1\n");
86         return 0;
87     }
88     printf("%d\n",cnt);
89     dfs(p[1].id);
90     return 0;
91 }