POJ - 3041 最小顶点覆盖

题意：

思路：

代码：

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

const int MAXN = 505;
const int MAXM = 505;

struct node {
int x, y, z;
bool operator < (const node &b) const {
if (x == b.x) {
if (y == b.y) return z < b.z;
return y < b.y;
}
return x < b.x;
}
}a[MAXN];

struct BgMaxMatch {

int nl, nr;
int l[MAXN], r[MAXM];
bool vis[MAXN];
vector <int> G[MAXN];  //这里注意要看是按左右哪边建图，也有可能是MAXM

void init(int nl, int nr) {
this -> nl = nl;
this -> nr = nr;
for (int i = 1; i <= nl; i++) G[i].clear();
memset(vis, false, sizeof(vis));
}

void AddEdge(int u, int v) {
G[u].push_back(v);
}

bool dfs(int u){
int cnt = G[u].size();
for (int i = 0; i < cnt; i++) {
int v = G[u][i];
if (!vis[v]) {
vis[v] = true;
if(l[v] == -1 || dfs(l[v]))	{
l[v] = u; r[u] = i;
return true;
}
}
}
return false;
}

int MaxMatch() {
memset(l, -1, sizeof(l));
memset(r, -1, sizeof(r));
int ans = 0;
for (int i = nl; i >= 1; i--) {
memset(vis, false, sizeof(vis));
ans += dfs(i);
}
return ans;
}
} solver;

int main() {
int n, k;
while (scanf("%d%d", &n, &k) != EOF) {
solver.init(n, n);
for (int i = 1; i <= k; i++) {
int x, y;
scanf("%d%d", &x, &y);
solver.AddEdge(x, y);
}
printf("%d\n", solver.MaxMatch());
}
return 0;
}