HDU 2444 The Accomodation of Students——二分图判定+最大匹配

判定后用匈牙利算法求出最大匹配

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
const int maxn = 500;
int n, m;
vector<int> G[maxn];
int vis[maxn];
int s1[maxn], s2[maxn], n1, n2;
bool used[maxn];
int match[maxn];

bool dfs(int u) {
    for (int i = 0; i < G[u].size(); i++) {
        int v = G[u][i];
        if (vis[v]) {
            if (vis[v] == -vis[u]) continue;
            else return false;
        }
        else {
            vis[v] = -vis[u];
            if (!dfs(v)) return false;
        }
    }
    return true;
}

bool Edmons(int u) {
    for (int i = 0; i < G[u].size(); i++) {
        int v = G[u][i];
        if(used[v]) continue;
        used[v] = true;
        if (match[v] == 0 || Edmons(match[v])) {
            match[v] = u;
            return true;
        }
    }
    return false;
}

int main() {
    while (~scanf("%d %d", &n, &m)) {
        int i;
        for (int i = 0; i <= n; i++) G[i].clear();
        for (i = 1; i <= m; i++) {
            int u, v; scanf("%d %d", &u, &v);
            G[u].push_back(v);
            G[v].push_back(u);
        }
        memset(vis, 0, sizeof(vis));
        for (i = 1; i <= n; i++) {
            if (vis[i]) continue;
            vis[i] = 1;
            if (!dfs(i)) break;
        }
        if (i <= n) printf("No\n");
        else {
            n1 = n2 = 0;
            for (int i = 1; i <= n; i++) {
                if (vis[i] == 1) s1[++n1] = i;
                else if (vis[i] == -1) s2[++n2] = i;
            }
            for (int i = 1; i <= n; i++) match[i] = 0;
            int ans = 0;
            for (int i = 1; i <= n1; i++) {
                int u = s1[i];
                memset(used, false, sizeof(used));
                if (Edmons(u)) ans++;
            }
            printf("%d\n", ans);
        }
    }
    return 0;
}