网络流24题

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
using namespace std;
const int Maxn = 110;
struct node {
	int y, next, c, opp;
}a[Maxn*Maxn*4]; int first[Maxn*2], len;
void ins(int x, int y, int c) {
	len++; int k1 = len;
	a[len].y = y; a[len].c = c;
	a[len].next = first[x]; first[x] = len;
	len++; int k2 = len;
	a[len].y = x; a[len].c = 0;
	a[len].next = first[y]; first[y] = len;
	a[k1].opp = k2;
	a[k2].opp = k1;
}
int st, ed, h[Maxn*2];
int n, m;
bool bfs() {
	queue <int> q;
	memset(h, -1, sizeof(h));
	h[st] = 0;
	q.push(st);
	while(!q.empty()){
		int x = q.front(); q.pop();
		for(int k = first[x]; k; k = a[k].next){
			int y = a[k].y;
			if(h[y] == -1 && a[k].c > 0){
				h[y] = h[x]+1;
				q.push(y);
			}
		}
	}
	return h[ed] > 0;
}
int _min(int x, int y) { return x < y ? x : y; }
int dfs(int x, int flow) {
	if(x == ed) return flow;
	int delta = 0;
	for(int k = first[x]; k; k = a[k].next){
		int y = a[k].y;
		if(h[y] == h[x]+1 && a[k].c > 0 && flow-delta > 0){
			int minf = dfs(y, _min(a[k].c, flow-delta));
			delta += minf;
			a[k].c -= minf;
			a[a[k].opp].c += minf;
		}
	}
	if(delta == 0) h[x] = -1;
	return delta;
}
int main() {
	int i, j, k;
	scanf("%d%d", &n, &m);
	st = 0; ed = n+1;
	int x, y;
	while(scanf("%d%d", &x, &y) != EOF){
		ins(x, y, 1);
	}
	for(i = 1; i <= m; i++) ins(st, i, 1);
	for(i = m+1; i <= n; i++) ins(i, ed, 1);
	int ans = 0;
	while(bfs()) ans += dfs(st, 0x7fffffff);
	printf("%d\n", ans);
	return 0;
}