洛谷 P1967 货车运输 java实现

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题目链接:
https://www.luogu.com.cn/problem/P1967
是一道最小生成树+LCA的模板组合题目.
代码比较长....

计算最多能运送多少重量的货物,首先会想到计算最大生成树.这样就能够保证联通所有路的情况下,道路载重最大.
因为最终计算的是某两点之间的最大载重量,所以我们需要计算两点之间的路径上最小的载重是多少.
在倍增法LCA中,我们通过动态规划可以计算出每个节点的跳跃路径,我们也可以使用同样的方式记录在跳跃过程中,道路的最小载重.

for (int j = 1; j <= maxLevel; j++) {
    for (int i = 1; i <= n; i++) {
        parentInfo[i][j] = parentInfo[parentInfo[i][j - 1]][j - 1];
        parentMinWeightInfo[i][j] = Math.min(parentMinWeightInfo[i][j-1],parentMinWeightInfo[parentInfo[i][j - 1]][j - 1]);
    }
}

最终,在进行询问的时候,可以进行比较,得出最终结果.

因为给出的点之间可能并不联通,是一个森林,所以在询问之前,需要进行联通判断.不在一个并查集中,就输出-1.

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.util.*;

public class Main {
    static int[] p, level;
    static PriorityQueue<int[]> pq;
    static LinkedList<int[]>[] adj;
    static boolean[] isVisit;
    static int[][] parentInfo,parentMinWeightInfo;
    static int maxLevel,n,m;

    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());
        n = Integer.parseInt(st.nextToken());
        m = Integer.parseInt(st.nextToken());

        p = new int[n + 1];
        pq = new PriorityQueue<>(new Comparator<int[]>() {
            @Override
            public int compare(int[] o1, int[] o2) {
                return o2[2] - o1[2];
            }
        });
        for (int i = 1; i <= n; i++) {
            p[i] = i;
        }
        int from, to, weight;

        for (int i = 1; i <= m; i++) {
            st = new StringTokenizer(br.readLine());
            from = Integer.parseInt(st.nextToken());
            to = Integer.parseInt(st.nextToken());
            weight = Integer.parseInt(st.nextToken());
            pq.add(new int[]{from, to, weight});
        }

        adj = new LinkedList[n + 1];
        for (int i = 0; i < adj.length; i++) {
            adj[i] = new LinkedList<int[]>();
        }

        int[] edge;
        while (!pq.isEmpty()) {
            edge = pq.poll();
            from = edge[0];
            to = edge[1];
            weight = edge[2];
            if (!connect(from, to)) {
                union(from, to);
                adj[from].add(new int[]{to, weight});
                adj[to].add(new int[]{from, weight});
            }
        }

        isVisit = new boolean[n + 1];
        level = new int[n + 1];
        for (int i = 1; i <= n; i++) {
            if (!isVisit[i]) {
                dfs(i,0);
            }
        }
        maxLevel = 0;
        for (int i = 1; i <= n; i++) {
            maxLevel = Math.max(maxLevel, level[i]);
        }
        int temp = 0;
        while ((1 << temp) < maxLevel) {
            temp++;
        }
        maxLevel = temp;
        parentInfo = new int[n + 1][maxLevel + 1];
        parentMinWeightInfo = new int[n + 1][maxLevel + 1];

        isVisit = new boolean[n + 1];
        for (int i = 1; i <= n; i++) {
            if (!isVisit[i]) {
                parentMinWeightInfo[i][0] = Integer.MAX_VALUE;
                dfs2(i);
            }
        }

        for (int j = 1; j <= maxLevel; j++) {
            for (int i = 1; i <= n; i++) {
                parentInfo[i][j] = parentInfo[parentInfo[i][j - 1]][j - 1];
                parentMinWeightInfo[i][j] = Math.min(parentMinWeightInfo[i][j-1],parentMinWeightInfo[parentInfo[i][j - 1]][j - 1]);
            }
        }

        st = new StringTokenizer(br.readLine());
        StringBuilder bu = new StringBuilder();
        int q = Integer.parseInt(st.nextToken());
        int a, b;
        for (int i = 0; i < q; i++) {
            st = new StringTokenizer(br.readLine());
            a = Integer.parseInt(st.nextToken());
            b = Integer.parseInt(st.nextToken());
            if(!connect(a,b)){
                bu.append(-1) ;
                if(i != q-1){
                    bu.append("\n");
                }
                continue;
            }
            int ans = LCA(a,b);
            bu.append(ans) ;
            if(i != q-1){
                bu.append("\n");
            }
        }
        System.out.println(bu.toString());
    }

    private static void dfs(int from,int deep) {
        isVisit[from] = true;
        level[from] = ++deep;
        for (int[] next : adj[from]) {
            if (!isVisit[next[0]]) {
                dfs(next[0],deep);
            }
        }
    }

    private static void dfs2(int from){
        isVisit[from] = true;
        for (int[] next : adj[from]) {
            if (!isVisit[next[0]]) {
                parentMinWeightInfo[next[0]][0] = next[1];
                parentInfo[next[0]][0] = from;
                dfs2(next[0]);
            }
        }
    }

    private static int LCA(int a, int b){
        if(level[a]<level[b]){
            int temp =a;
            a = b;
            b = temp;
        }
        int ans = Integer.MAX_VALUE;
        for(int i = maxLevel;i>=0;i--){
            if(level[parentInfo[a][i]]>=level[b]){
                ans = Math.min(ans, parentMinWeightInfo[a][i]);
                a = parentInfo[a][i];
            }
        }
        if(a == b){
            return ans;
        }
        for(int i = maxLevel;i>=0;i--){
            if(parentInfo[a][i]!=parentInfo[b][i]){
                ans = Math.min(ans,Math.min(parentMinWeightInfo[a][i],parentMinWeightInfo[b][i]));
                a = parentInfo[a][i];
                b = parentInfo[b][i];
            }
        }
        ans = Math.min(ans,Math.min(parentMinWeightInfo[a][0],parentMinWeightInfo[b][0]));
        return ans;
    }

    private static int find(int a) {
        if (p[a] == a) {
            return p[a];
        }
        return p[a] = find(p[a]);
    }

    private static void union(int a, int b) {
        int A = find(a);
        int B = find(b);
        p[A] = B;
    }

    private static boolean connect(int a, int b) {
        return find(a) == find(b);
    }
}

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