https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=2021

题解：二分图判断

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
#define endl "\n"
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,p,l,r,u,v;
int ans,cnt[2],flag,temp,sum;
int col[N];
char str;
struct node{};
vector<int>G[N];
bool dfs(int u,int color){
    col[u]=color;
    cnt[color]++;
    for(int i=0,j=G[u].size();i<j;i++){
        int v=G[u][i];
        if(col[u]==col[v])
            return 0;
        if(col[v]==-1&&!dfs(v,!color))
            return 0;
    }
    return 1;
}
void init(){
    for(int i=0;i<n;i++){
        G[i].clear();
    }
    memset(col,-1,sizeof(col));

    ans=1;
    sum=0;
}
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    scanf("%d",&t);
    while(t--){
        scanf("%d%d",&n,&m);
        init();
        for(int i=1;i<=m;i++){
            scanf("%d%d",&u,&v);
            G[u].push_back(v);
            G[v].push_back(u);
        }
        for(int i=0;i<n;i++){
            if(col[i]==-1){
                memset(cnt,0,sizeof(cnt));
                ans&=dfs(i,1);
                if(ans){
                    if(cnt[0]+cnt[1]==1){
                        sum++;
                    }else{
                        sum+=min(cnt[0],cnt[1]);
                    }
                }
            }
        }
        if(ans){
            cout<<sum<<endl;
        }else{
            cout<<-1<<endl;
        }
    }

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}