參考：

https://blog.csdn.net/cillyb/article/details/55511666

https://blog.csdn.net/c20180630/article/details/70175814

模板：

#include <bits/stdc++.h>
using namespace std;
const int maxN = 1e5 + 7;
vector<int> G[maxN];
int match[maxN];
int vis[maxN];
int n, m, sum;
int dfs(int u) {

    for(int i = 0; i < G[u].size(); i++) {
        
 
int v = G[u][i];
        //有路而且沒被訪問
        if(!vis[v]) {
            vis[v] = 1;//標記點i已經訪問過

            //如果點i未被配對或者找到了新的配對
            if(match[v] == 0 || dfs(match[v])) {
                //更新配對關系
                match[v] = u;
                match[u] = v;
                return 1;
            }
        }
    }
    
 
return 0;
}

int main() {
//    freopen("1.txt","r", stdin);
    scanf("%d %d", &n, &m);
    for(int i = 1; i <= n; i++) {
        int u, v;
        scanf("%d %d", &u, &v);
        G[u].push_back(v);
        G[v].push_back(u);
    }

    memset(match, 0 , sizeof(match));

    for(int i = 1
 
; i <= n; i++){
        memset(vis, 0, sizeof(vis));
        if(dfs(i)) {
            sum++;
        }
    }
    printf("%d\n", sum);


}

二分圖最大匹配（匈牙利算法）