### onlyone's blog

By onlyone, 13 years ago, Problem A:
Check whether exist a pair i and j,they satisfy xi+di = xj && xj+dj = xi;

Problem B:
Pay attention to Just Right or Red. use Div and Mod can solve it easily;

Problem C:
As we know, there are only two path -- forward and reverse, so we can do DFS from the one-degree nodes (only two nodes).
As their index may be very larger, so I used map<int,int> to do hash.

void dfs(int i)
{
int sz = mat[i].size()-1, j;
UF(j,0,sz)
if (!vis[mat[i][j]])
{
vis[mat[i][j]] = 1;
printf(" %d",val[mat[i][j]]);
dfs(mat[i][j]);
}
}

Problem D:
Floyd.
First, Floyd pretreat the path from I to J, and save the path.
The order is a1,a2...ak, K is the number of the leaves, we can assume a0 = ak+1 = 1, the root.

if the ans.size() > 2*N-1 , cout -1;   Comments (16)
| Write comment?
•  first, i use floyd save all the path[i][j], vector path[][];path[i][j], from i to j;then, just push all the path, from 1 to next leaf, and come back 1.
•  full solution using lca, better complexity
•  Same complexity without LCA. For each leaf y let leaf_pos[y] be the position of the leaf in the desired visit order. For each node x let dp[x] be min value of any leaf_pos for all leaves rooted in the subtree of x. Then do a dfs visiting nodes in order of increasing value of dp. 92775116
•  wow, thats smart, very clean code. i think its important to solve problems, using different approaches. learned something new, thanks
•  if (sum != 2 * n — 2) { fine = 0; }what is the meaning of this line in your code?plz explain
•  there are n — 1 edges in a tree, in the problem you are asked to find path from root to root such that you go through each edge only two times, thus total walked distance is 2 * (n — 1) = 2 * n — 2.
•  Thanks a lot,sir!
 » About problem D. Since the route between two nodes in a tree can be split into two parts by their least common ancestor, I think maybe a better solution would using directly walk between two adjacent nodes by passing their lca and connect the edges.Although, since n is small enough, Floyd is just good for it. :-)
 » can anyone post the idea for 29E
•  » » It's a shortest path problem. The state is u, v, t, which means person 0 is at vertex u, person 1 is at vertex v, and person t must move now. For t = 1, you have to check that the vertex you move to is different from u. You can solve it with BFS.
•  » » » Thank you very much
•  » » » Thanks. That was really helpful.
 » why should we use map ?
•  » » Assuming the question is for C, the range of numbers is from 1 to 10^9 which means to store graph you will use an array of size 10^9 which is beyond the memory limit of your hardware.
 » fuckoff @onlyone is this editorial or a Treasure hunting letter??.