題意：構造一棵樹，有N個點，直徑為d，深度為h

思路：首先構造一個長度為d的鏈，然后把其中一個距離邊上為h的點變為根。然后我們就不停的在距離根為h上面的那一點不停的加點就好了，使得新加入的點的距離也為h。不過坑點挺多的....

#include<bits/stdc++.h> using namespace std; const int maxn = 1e6+7; int n,d,h; int ans1[maxn],ans2[maxn]; int tot = 0; int idx[maxn]; vector<int> T; int now = 0; void addans(int x,int y) {ans1[tot]=x,ans2[tot]=y;tot++; } int main() {scanf("%d%d%d",&n,&d,&h);if(d>2*h)return puts("-1"),0;if(d==1&&n>2)return puts("-1"),0;if(d==2&&h==1){for(int i=2;i<=n;i++)addans(1,i);for(int i=0;i<tot;i++)printf("%d %d\n",ans1[i],ans2[i]);}else{for(int i=2;i<=n;i++)T.push_back(i);for(int i=1;i<=h;i++)idx[i]=T[now++];idx[h+1]=1;for(int i=h+2;i<=d+1;i++)idx[i]=T[now++];for(int i=1;i<=d;i++)addans(idx[i],idx[i+1]);for(int i=now;i<T.size();i++)addans(T[i],idx[h]);for(int i=0;i<tot;i++)printf("%d %d\n",ans1[i],ans2[i]);}}

Description

A tree is a connected undirected graph consisting of?n?vertices and?n??-??1?edges. Vertices are numbered?1?through?n.

Limak is a little polar bear and Radewoosh is his evil enemy. Limak once had a tree but Radewoosh stolen it. Bear is very sad now because he doesn't remember much about the tree?— he can tell you only three values?n,?d?and?h:

• The tree had exactly?n?vertices.
• The tree had diameter?d. In other words,?d?was the biggest distance between two vertices.
• Limak also remembers that he once rooted the tree in vertex?1?and after that its height was?h. In other words,?h?was the biggest distance between vertex?1?and some other vertex.

The distance between two vertices of the tree is the number of edges on the simple path between them.

Help Limak to restore his tree. Check whether there exists a tree satisfying the given conditions. Find any such tree and print its edges in any order. It's also possible that Limak made a mistake and there is no suitable tree?– in this case print "-1".

Input

The first line contains three integers?n,?d?and?h?(2?≤?n?≤?100?000,?1?≤?h?≤?d?≤?n?-?1)?— the number of vertices, diameter, and height after rooting in vertex?1, respectively.

Output

If there is no tree matching what Limak remembers, print the only line with "-1" (without the quotes).

Otherwise, describe any tree matching Limak's description. Print?n?-?1?lines, each with two space-separated integers?– indices of vertices connected by an edge. If there are many valid trees, print any of them. You can print edges in any order.

Sample Input

Input 5 3 2 Output 1 2 1 3 3 4 3 5 Input 8 5 2 Output -1 Input 8 4 2 Output 4 8 5 7 2 3 8 1 2 1 5 6 1 5

轉載于:https://www.cnblogs.com/q934098774/p/5388711.html

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