标准的二分图判定，没什么好说的（很久以前打的，很丑）
1 #include <bits/stdc++.h> 2 #define read read() 3 #define up(i,l,r) for(register int i = (l);i <= (r);i++) 4 #define down(i,l,r) for(register int i = (l);i >= (r);i--) 5 #define traversal_vedge(i) for(register int i = head[u]; i ;i = e[i].nxt) 6 #define ll long long 7 using namespace std; 8 int read 9 { 10 int x = 0, f = 1; char ch = getchar(); 11 while(ch < 48 || ch > 57) {if(ch == '-')f = -1; ch = getchar();} 12 while(ch >=48 && ch <=57) {x = 10 * x + ch - 48;ch = getchar();} 13 return x * f; 14 } 15 const int N = 1e5 + 5; 16 struct edge{ 17 int v,nxt; 18 }e[N<<1];int size,head[N]; 19 20 bool vis[N],color[N]; 21 int n,m,ans,sum[2]; 22 23 void add(int u,int v) { e[++size] = (edge){v,head[u]}; head[u] = size; } 24 25 bool dfs(int u,int col) 26 { 27 if(vis[u]) 28 { 29 if(color[u] == col) return true; 30 else printf("Impossible "),exit(0); 31 } 32 vis[u] = 1; 33 color[u] = col; 34 sum[col]++; 35 traversal_vedge(i) 36 { 37 int v = e[i].v; 38 dfs(v,col^1); 39 } 40 return true; 41 } 42 43 int main() 44 { 45 n = read; m = read; 46 int u,v; 47 up(i,1,m) 48 { 49 u = read; v = read; 50 add(u,v); 51 add(v,u); 52 } 53 up(i,1,n) 54 { 55 sum[0] = sum[1] = 0; 56 if(vis[i]) continue; 57 dfs(i,1); 58 ans += min(sum[0],sum[1]); 59 } 60 printf("%d ",ans); 61 return 0; 62 }