POJ 3694 Network

tarjan+LCA。
先用tarjan进行缩点。
然后用dfn[u] < low[v]判断是不是桥。
讲桥标记起来并且计算出桥数。
对于每个案例的u和v
利用LCA用fath数组先找到u和v的祖先，如果在路径中出现的桥，就把桥标记成普通边并将桥数–。
然后输出对应的桥数

#include<stdio.h>
#include<string.h>
#include<algorithm>

using namespace std;

const int maxn = 100006;
const int maxm = 200005;
int n, m, tol, cnt;
int brinum;

struct Node {
    int u;
    int v;
    int next;
};
Node node[2 * maxm];
int dfn[maxn];
int low[maxn];
int fath[maxn];
int head[maxn];
bool vis[maxn];
bool bridge[maxn];

void init() {
    brinum = tol = cnt = 0;
    memset(dfn, 0, sizeof(dfn));
    memset(low, 0, sizeof(low));
    memset(node, 0, sizeof(node));
    memset(fath, 0, sizeof(fath));
    memset(head, -1, sizeof(head));
    memset(vis, false, sizeof(vis));
    memset(bridge, 0, sizeof(bridge));
}

void addnode(int u, int v) {
    node[tol].u = u;
    node[tol].v = v;
    node[tol].next = head[u];
    head[u] = tol++;
}

void dfs(int u, int fa) {
    dfn[u] = low[u] = ++cnt;
    fath[u] = fa;
    for(int i=head[u]; i != -1; i=node[i].next) {
        int v = node[i].v;
        if(!dfn[v]) {
            dfs(v, u);
            low[u] = min(low[u], low[v]);
        } else if(v != fa) {
            low[u] = min(low[u], dfn[v]);
        }
    }
}

void tarjan() {
    dfs(1, 1);
}

void findbridge() {
    for(int v=1; v<=n; v++) {
        int u = fath[v];
        if(dfn[u] < low[v] && u != v) {
            bridge[v] = true;
            brinum++;
        }
    }
}

void LCA(int u, int v) {
    while(dfn[u] > dfn[v]) {
        if(bridge[u]) {
            brinum--;
            bridge[u] = false;
        }
        u = fath[u];
    }
    while(dfn[v] > dfn[u]) {
        if(bridge[v]) {
            brinum--;
            bridge[v] = false;
        }                                   
        v = fath[v];
    }
    while(u != v) {
        if(bridge[u]) {
            bridge[u] = false;
            brinum--;
        }
        if(bridge[v]) {
            bridge[v] = false;
            brinum--;
        }
        u = fath[u];
        v = fath[v];
    }
    printf("%d\n", brinum);
}

int main() {
    int cas = 1;
    while(scanf("%d%d",&n, &m), n||m) {
        init();
        for(int i=1; i<=m; i++) {
            int u, v;
            scanf("%d%d", &u, &v);
            addnode(u, v);
            addnode(v, u);
        }
        tarjan();
        findbridge();
        int q;
        scanf("%d", &q);
        printf("Case %d:\n", cas++);
        for(int i=1; i<=q; i++) {
            int u, v;
            scanf("%d%d", &u, &v);
            LCA(u, v);
        }
        puts("");
    }
    return 0;
}