leetcode 310 : Minimum Height Trees

1、原题如下：

For a undirected graph with tree characteristics, we can choose any node as the root. The result graph is then a rooted tree. Among all possible rooted trees, those with minimum height are called minimum height trees (MHTs). Given such a graph, write a function to find all the MHTs and return a list of their root labels.

Format
The graph contains n nodes which are labeled from 0 to n - 1. You will be given the number n and a list of undirected edges (each edge is a pair of labels).

You can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.

Example 1:

Given n = 4, edges = [[1, 0], [1, 2], [1, 3]]

    0    |    1   / \  2   3

return [1]

Example 2:

Given n = 6, edges = [[0, 3], [1, 3], [2, 3], [4, 3], [5, 4]]

 0  1  2  \ | /    3    |    4    |    5

return [3, 4]

Note:

(1) According to the definition of tree on Wikipedia: “a tree is an undirected graph in which any two vertices are connected by exactly one path. In other words, any connected graph without simple cycles is a tree.”

(2) The height of a rooted tree is the number of edges on the longest downward path between the root and a leaf.

2、解题如下：

class Solution {public:    struct vertex    {        unordered_set<int> neighbor;        bool isLeaf() const{return neighbor.size()==1;}    };    vector<int> findMinHeightTrees(int n, vector<pair<int, int>>& edges) {        vector<int> tmp1;        vector<int> tmp2;        vector<int>* tmp1_1=&tmp1;        vector<int>* tmp2_1=&tmp2;        if(n==1)        {            tmp1.push_back(0);            return tmp1;        }        if(n==2)        {            tmp1.push_back(0);            tmp1.push_back(1);            return tmp1;        }        vector<vertex> vertices(n);        for(auto i:edges)        {            vertices[i.first].neighbor.insert(i.second);            vertices[i.second].neighbor.insert(i.first);        }        for(int i=0;i<n;i++)        {            if(vertices[i].isLeaf())            {                tmp1_1->push_back(i);            }        }        while(1)        {            for(auto j:*tmp1_1)            {                for(auto k: vertices[j].neighbor)                {                    vertices[k].neighbor.erase(j);                    if(vertices[k].isLeaf()) tmp2_1->push_back(k);                }            }            if(tmp2_1->empty())            {                return *tmp1_1;            }            tmp1_1->clear();            swap(tmp1_1,tmp2_1);        }    }};