judge loop in undirected graph

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一深度优先遍历，参考前面DFS(white and gray and black)

二根据定点以及边数目进行判断

如果m(edge)大于n(vertex),那么肯定存在环

算法如下：

1 删除所有入度小于等于1的顶点，并且将和这些顶点相关的顶点入度减1

2 将入度变为1的顶点全部删除，重复上述动作，如果最后还有顶点那么图中存在环

具体代码如下：

#include <iostream>using namespace std;#define MAX_VERTEX_NUM 128enum color{WHITE, GRAY = 1, BLACK};bool M[MAX_VERTEX_NUM][MAX_VERTEX_NUM];int colour[MAX_VERTEX_NUM];int dfsNum[MAX_VERTEX_NUM], num;int indegree[MAX_VERTEX_NUM];int vexnum, edgenum;void init_graph(){    cout<<"enter vertex number:"<<endl;    cin>>vexnum;    cout<<"enter edge number:"<<endl;    cin>>edgenum;    int i, j;    while(edgenum){        cout<<"add new edge:"<<endl;        cin>>i>>j;        M[i - 1][j - 1] = true;        //initialize in vertex degree        indegree[i - 1]++;        indegree[j - 1]++;        edgenum--;    }}/*void dfs(int u, int p){    colour[u] = GRAY;    dfsNum[u] = num++;    for( int v = 0; v < vexnum; v++){        if(M[u][v] && v != p){            if(colour[v] == WHITE) dfs(v, u);            else if(colour[v] == GRAY)                cout<<"back edge between"<<u + 1<<" and"<<v + 1<<endl;            else if(colour[v] == BLACK)                cout<<"cross edge between"<<u + 1<<" and"<<v + 1<<endl;;        }    }    colour[u] = BLACK;}void print_dfs_num(){    for(int v = 0; v < vexnum; v++)        cout<<dfsNum[v]<<" ";}*/void LoopJudge(){    bool loop = false;    int twice = 2;    int k, i, j;    cout<<"line: "<<__LINE__<<endl;    for( k = twice; k > 0; k--){        cout<<"line: "<<__LINE__<<"k: "<<k<<endl;        for( i = 0; i < vexnum; i++){            cout<<"line: "<<__LINE__<<"i: "<<i<<endl;            if(indegree[i] <= 1){                indegree[i] = 0;   //delete vertex in degree equal one                for( j = 0; j < vexnum; j++){                     cout<<"line: "<<__LINE__<<"j: "<<j<<endl;                    if(M[i][j]){                        M[i][j] = false;                        M[j][i] = false;                        indegree[j]--;                        }//if(M[i][j])                }//for(int j = 0; j < vexnum; j++)            }//if(indegree[i] <= 1)        }//for(int i = 0; i < vexnum; i++)    }      for( k = 0; k < vexnum; k++){            if(indegree[k] != 0){                loop = true;            }      }      if(loop)        cout<<"There is loop in undirected graph!"<<endl;      else        cout<<"There is no loop in undirected graph!"<<endl;}int main(){    init_graph();    //dfs(0, -1);    //print_dfs_num();    LoopJudge();    int ch;    cin>>ch;    return 0;}

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