hdu 4612 Warm up（缩点+树直径）

解题思路

代码

#include <cstdio>
#include <cstring>
#include <queue>
#include <algorithm>

using namespace std;

const int maxn = 200005;
const int maxm = 1000005;

int N, M, E, first[maxn], jump[maxm<<1], linker[maxm<<1];
int dfsclock, L[maxm], R[maxm], iscut[maxm], pre[maxn];

int dfs (int u, int fa) {
int lowu = pre[u] = ++dfsclock;
for (int i = first[u]; i + 1; i = jump[i]) {
int v = linker[i];
if (!pre[v]) {
int lowv = dfs(v, u);
lowu = min(lowu, lowv);
if (lowv > pre[u])
iscut[i>>1] = 1;
} else if (pre[v] < pre[u] && v != fa)
lowu = min(lowu, pre[v]);
}
return lowu;
}

void dfs (int u, int fa, int be) {
pre[u] = be;
for (int i = first[u]; i + 1; i = jump[i]) {
int v = linker[i];
if (pre[v] || iscut[i>>1]) continue;
dfs(v, u, be);
}
}

void findEdge () {
dfsclock = 0;
memset(pre, 0, sizeof(pre));
memset(iscut, 0, sizeof(iscut));
for (int i = 1; i <= N; i++)
if (!pre[i]) dfs(i, -1);

dfsclock = 0;
memset(pre, 0, sizeof(pre));
for (int i = 1; i <= N; i++)
if (!pre[i]) dfs(i, -1, ++dfsclock);
}

void addEdge(int u, int v) {
jump[E] = first[u];
linker[E] = v;
first[u] = E++;
}

void init () {
E = 0;
memset(first, -1, sizeof(first));

for (int i = 0; i < M; i++) {
scanf("%d%d", &L[i], &R[i]);
addEdge(L[i], R[i]);
addEdge(R[i], L[i]);
}
findEdge();
}

const int inf = 0x3f3f3f3f;
int D[maxn];

int solve () {
int n = dfsclock, u;

queue<int> que;
que.push(1);

memset(D, inf, sizeof(D));
D[1] = 0;
while (!que.empty()) {
u = que.front();
que.pop();
for (int i = first[u]; i + 1; i = jump[i]) {
int v = linker[i];
if (D[v] > D[u] + 1) {
D[v] = D[u] + 1;
que.push(v);
}
}
}

que.push(u);
memset(D, inf, sizeof(D));
D[u] = 0;
while (!que.empty()) {
u = que.front();
que.pop();
for (int i = first[u]; i + 1; i = jump[i]) {
int v = linker[i];
if (D[v] > D[u] + 1) {
D[v] = D[u] + 1;
que.push(v);
}
}
}
return n - D[u] - 1;
}

int main () {
while (scanf("%d%d", &N, &M) == 2 && N + M) {
init();

E = 0;
memset(first, -1, sizeof(first));
for (int i = 0; i < M; i++) if (iscut[i]) {
addEdge(pre[L[i]], pre[R[i]]);
addEdge(pre[R[i]], pre[L[i]]);
}
printf("%d\n", solve());
}
return 0;
}