【二分图匹配】Plug It In!

http://codeforces.com/gym/101873
F

先对原图跑一遍匈牙利得到原始最大匹配，再遍历每个出度>1的点，考虑若新加入点，能否找到增广路，若可行则答案对应增加

代码：

#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;
typedef long long ll;
const int MAX_V = 2000;
const int MAX_E = 80000;
struct Hungary {
    struct Edge {
        int to, next;
    } es[MAX_E];
    int head[MAX_V];
    int V, E, match[MAX_V];
    bool vis[MAX_V];

    void init(int V) {
        this->V = V;
        this->E = 0;
        memset(head, -1, sizeof head);
    }

    void addEdge(int u, int v) {
        es[E].to = v;
        es[E].next = head[u];
        head[u] = E++;
    }

    bool dfs(int u) {
        vis[u] = true;
        for (int i = head[u]; i != -1; i = es[i].next) {
            int v = es[i].to, w = match[v];
            if (w < 0 || !vis[w] && dfs(w)) {
                match[v] = u;
                return true;
            }
        }
        return false;
    }

    int maxMatch() {
        int res = 0;
        memset(match, -1, sizeof match);
        for (int u = 0; u < V; u++) {
            memset(vis, false, sizeof vis);
            if (dfs(u)) {
                res++;
            }
        }
        return res;
    }
} hun;
int main() {
    int m, n, k, out[2000] = { 0 }, match[2000] = { 0 };
    scanf("%d%d%d", &m, &n, &k);
    hun.init(m);
    for (int i = 0; i < k; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        hun.addEdge(u - 1, v - 1);
        out[u - 1]++;
    }
    int res = hun.maxMatch();
    int ans = res;
    memcpy(match, hun.match, sizeof hun.match);
    for (int u = 0; u < m; u++) {
        if (out[u] > 1) {
            memcpy(hun.match, match, sizeof match);
            int cnt = 0;
            for (int j = 0; j < 2; j++) {
                hun.head[m + j] = -1;
                for (int i = hun.head[u]; ~i; i = hun.es[i].next) {
                    int v = hun.es[i].to;
                    hun.addEdge(m + j, v);
                    cnt++;
                }
            }
            hun.E -= cnt;
            int extra = 0;
            for (int j = 0; j < 2; j++) {
                memset(hun.vis, 0, sizeof hun.vis);
                if (hun.dfs(m + j)) {
                    extra++;
                }
            }
            ans = max(ans, res + extra);
        }
    }
    printf("%d\n", ans);
}