[ICPC 2020 Nanjing R] Degree of Spanning Tree

Luogu
IDLGP9625
Time2000ms
Memory256MB
DifficultyP6
图论2020Special JudgeO2优化构造ICPC南京
Given an undirected connected graph with $n$ vertices and $m$ edges, your task is to find a spanning tree of the graph such that for every vertex in the spanning tree its degree is not larger than $\frac{n}{2}$. Recall that the degree of a vertex is the number of edges it is connected to. ## Input There are multiple test cases. The first line of the input contains an integer $T$ indicating the number of test cases. For each test case: The first line contains two integers $n$ and $m$ ($2 \le n \le 10^5$, $n-1 \le m \le 2 \times 10^5$) indicating the number of vertices and edges in the graph. For the following $m$ lines, the $i$-th line contains two integers $u_i$ and $v_i$ ($1 \le u_i, v_i \le n$) indicating that there is an edge connecting vertex $u_i$ and $v_i$. Please note that there might be self loops or multiple edges. It's guaranteed that the given graph is connected. It's also guaranteed that the sum of $n$ of all test cases will not exceed $5 \times 10^5$, also the sum of $m$ of all test cases will not exceed $10^6$. ## Output For each test case, if such spanning tree exists first output ``Yes`` in one line, then for the following $(n-1)$ lines print two integers $p_i$ and $q_i$ on the $i$-th line separated by one space, indicating that there is an edge connecting vertex $p_i$ and $q_i$ in the spanning tree. If no valid spanning tree exists just output ``No`` in one line. [samples] ## Note ### Note For the first sample test case, the maximum degree among all vertices in the spanning tree is 3 (both vertex 1 and vertex 4 has a degree of 3). As $3 \le \frac{6}{2}$ this is a valid answer. For the second sample test case, it's obvious that any spanning tree will have a vertex with degree of 2, as $2 > \frac{3}{2}$ no valid answer exists.
Samples
Input #1
2
6 9
1 2
1 3
1 4
2 3
2 4
3 4
4 5
4 6
4 6
3 4
1 3
2 3
3 3
1 2
Output #1
Yes
1 2
1 3
1 4
4 5
4 6
No
API Response (JSON)
{
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    "name": "[ICPC 2020 Nanjing R] Degree of Spanning Tree",
    "description": {
      "content": "Given an undirected connected graph with $n$ vertices and $m$ edges, your task is to find a spanning tree of the graph such that for every vertex in the spanning tree its degree is not larger than $\\f",
      "description_type": "Markdown"
    },
    "platform": "Luogu",
    "limit": {
      "time_limit": 2000,
      "memory_limit": 262144
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    "difficulty": {
      "LuoguStyle": "P6"
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    "is_remote": true,
    "is_sync": true,
    "sync_url": null,
    "sign": "LGP9625"
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  "statements": [
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      "statement_type": "Markdown",
      "content": "Given an undirected connected graph with $n$ vertices and $m$ edges, your task is to find a spanning tree of the graph such that for every vertex in the spanning tree its degree is not larger than $\\f...",
      "is_translate": false,
      "language": "English"
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