Zoj 3820 building fire stations (Binary + BFS)

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Given a tree, select two gas stations and ask the minimum value of the distance between all points and the gas station, and output a method of gas station establishment.

Solution: The second distance is used to determine the complexity of the function is O (n). The complexity should be O (nlogn). Even if the constant coefficient is too large, the system ran 4.5 S, it's also drunk.
The judgment function performs BFS three times, but each BFS node traverses a maximum of one time. First, you can create a tree as the root node and find the node with the largest depth, create a gas station at the node at the location of L (determined ans) to move up, and build a tree with this node as the root. This is 2nd BFs, then, the deepest node is moved up to the node for 1 time to establish a gas station and determine whether the distance between the two gas stations is less than or equal to the value of L.

#include <cstdio>#include <cstring>#include <queue>#include <algorithm>using namespace std;const int maxn = 200005;const int INF = 0x3f3f3f3f;int M, first[maxn], jump[maxn * 2], edge[maxn * 2];int N, s, e, far[maxn], dis[maxn];queue<int> que;inline void add_edge(int u, int v) {    edge[M] = v;    jump[M] = first[u];    first[u] = M++;}void init () {    int l, r;    M = 0;    scanf("%d", &N);    memset(first, -1, sizeof(first));    for (int i = 1; i < N; i++) {        scanf("%d%d", &l, &r);        add_edge(l, r);        add_edge(r, l);    }}int bfs(int s) {    que.push(s);    dis[s] = far[s] = 0;    int u;    while (!que.empty()) {        u = que.front();        que.pop();        for (int i = first[u]; i + 1; i = jump[i]) {            int v = edge[i];            if (dis[v] > dis[u] + 1) {                far[v] = u;                dis[v] = dis[u] + 1;                que.push(v);            }        }    }    return u;}bool judge (int L) {    memset(dis, INF, sizeof(dis));    s = bfs(1);    for (int i = 0; i < L; i++) {        s = far[s];        if (far[s] == 0) {            e = s % N + 1;            return true;        }    }    memset(dis, INF, sizeof(dis));    e = bfs(s);    for (int i = 0; i < L; i++) {        if (far[e] == 0)            return true;        e = far[e];    }    if (e == s)        e = e % N + 1;    bfs(e);    for (int i = 1; i <= N; i++)        if (dis[i] > L)            return false;    return true;}void solve () {    if (N == 2) {        printf("0 1 2\n");        return ;    }    int l = 0, r = N;    while (l < r) {        int mid = (l + r) / 2;        if (judge(mid))            r = mid;        else            l = mid + 1;    }    judge(l);    printf("%d %d %d\n", l, s, e);}int main () {    int cas;    scanf("%d", &cas);    while (cas--) {        init();        solve();    }    return 0;}

Zoj 3820 building fire stations (Binary + BFS)