POJ 2723

逻辑错误的代码

//想了很久，发现自己做这道题时犯了一个大BUG。我的思路是，把一组钥匙看成一对X,-X。把
//门确定的关系连边。其实这样是有错的，因为边的意义是“必须”，而实际上，门确定的只是矛盾
//关系。不是必须是，不是门上锁其中一个选或不选就能影响到另一个。 

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

const int MAXN=3200;
const int MAXM=8000;

int bel[MAXN],head[MAXN],dfn[MAXN],low[MAXN],st[MAXN],stop,index,tot;
int belong[MAXN],pat;
bool stack[MAXN];

struct {
    int u,v;
    int next,Id;
}edge[MAXM];
int n,m;

void addedge(int u,int v){
    edge[tot].u=u;
    edge[tot].v=v;
    edge[tot].Id=tot;
    edge[tot].next=head[u];
    head[u]=tot++;
}

void init(){
    for(int i=0;i<n*2;i++){
        dfn[i]=low[i]=0;
        stack[i]=false;
        belong[i]=-1;
    }
    stop=0; index=0; pat=-1;
}

void tarjan(int u,int limited){
    int v;
    dfn[u]=low[u]=++index;
    st[stop++]=u;
    stack[u]=true;
    for(int e=head[u];e!=-1;e=edge[e].next){
        if(edge[e].Id<=limited){
            v=edge[e].v;
            if(dfn[v]==0){
                tarjan(v,limited);
                low[u]=min(low[u],low[v]);
            }
            else if(stack[v]){
                low[u] = min(low[u], dfn[v]);
            }
        }
    }
    if(dfn[u]==low[u]){
        pat++;
        do{
            v=st[--stop];
            stack[v]=false;
            belong[v]=pat;
        }while(u!=v);
    }
}

bool slove(int mid){
    init();
    for(int i=0;i<2*n;i++){
        if(dfn[i]==0){
            tarjan(i,mid);
        }
    }
    bool flag=true;
    for(int i=0;i<n;i++){
        if(belong[i]==belong[2*i+1]){
            flag=false;
            break;
        }
    }
    return flag;
}

int main(){
    int u,v,t; int s1,s2;
    while(scanf("%d%d",&n,&m)!=EOF){
        if(n==0&&m==0) break;
        for(int i=0;i<=2*n;i++)
        head[i]=-1;
        tot=1;
        for(int i=0;i<n;i++){
            scanf("%d%d",&u,&v);
            bel[u]=2*i; bel[v]=2*i+1;
        }
        for(int i=1;i<=m;i++){
            scanf("%d%d",&u,&v);
            s1=bel[u]; s2=bel[v];
            addedge(s1^1,s2);
            addedge(s2^1,s1);
        }
        int lown,high,mid;
        lown=1;high=m; int ans;
        while(lown<=high){
            mid=(lown+high)/2;
            if(slove(mid*2)){
            //    cout<<mid<<endl;
                ans=mid; lown=mid+1;
            }
            else high=mid-1;
        }
        printf("%d
",ans);
    }
    return 0;
}

别人的代码。已明白错在哪里。。。只是不想写了。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
#define MAXN 5005
#define MAXM 50005
#define INF 1000000000
using namespace std;
struct Edge
{
    int v, next;
}edge[MAXM];
int n, m, e;
int top, scc, index;
int low[MAXN], dfn[MAXN], instack[MAXN];
int head[MAXN], st[MAXN], fa[MAXN];
int a[MAXN], b[MAXN], hash[MAXN];
int x[MAXN], y[MAXN];
void init()
{
    top = scc = index = e = 0;
    memset(head, -1, sizeof(head));
    memset(dfn, 0, sizeof(dfn));
    memset(instack, 0, sizeof(instack));
}
void insert(int x, int y)
{
    edge[e].v = y;
    edge[e].next = head[x];
    head[x] = e++;
}
void tarjan(int u)
{
    low[u] = dfn[u] = ++index;
    instack[u] = 1;
    st[++top] = u;
    int v;
    for(int i = head[u]; i != -1; i = edge[i].next)
    {
        v = edge[i].v;
        if(!dfn[v])
        {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else if(instack[v]) low[u] = min(low[u], dfn[v]);
    }
    if(dfn[u] == low[u])
    {
        scc++;
        do
        {
            v = st[top--];
            instack[v] = 0;
            fa[v] = scc;
        }while(v != u);
    }
}
bool check(int n)
{
    for(int i = 1; i <= n; i++)
        if(fa[i] == fa[i + n]) return false;
    return true;
}
void build(int bound)
{
    init();
    for(int i = 1; i <= n; i++)
    {
        insert(a[i], b[i] + 2 * n);
        insert(b[i], a[i] + 2 * n);
    }
    for(int i = 1; i <= bound; i++)
    {
        insert(x[i] + 2 * n, y[i]);
        insert(y[i] + 2 * n, x[i]);
    }
    for(int i = 1; i <= 2 * n; i++)
        if(!dfn[i]) tarjan(i);
}
void solve()
{
    int ans = 0;
    int low = 0, high = m;
    while(low <= high)
    {
        int mid = (low + high) >> 1;
        build(mid);
        if(check(n * 2)) ans = max(ans, mid), low = mid + 1;
        else high = mid - 1;
    }
    printf("%d
", ans);
}
int main()
{
    while(scanf("%d%d", &n, &m) != EOF)
    {
        if(n == 0 && m == 0) break;
        for(int i = 1; i <= n; i++)
        {
            scanf("%d%d", &a[i], &b[i]);
            a[i]++; b[i]++;
        }
        for(int i = 1; i <= m; i++)
        {
            scanf("%d%d", &x[i], &y[i]);
            x[i]++; y[i]++;
        }
        solve();
    }
    return 0;
}