图

顶点 边 无向图 度 有向图 入度 出度 待权图

存储方法:

1. 邻接矩阵
2. 邻接表

微博好友关系查找 内存 邻接表 逆邻接表 +红黑树 分治算法 硬盘存储 db uid toUid建立双向索引

深度搜索(Breadth-First-Searc)

广度搜索(Depth-First-Search)

public class Graph {
    private int v;
    private LinkedList<Integer> adj[]; // 邻接表
    private boolean found = false;

    public Graph(int v) {
        this.v = v;
        adj = new LinkedList[v];
        for (int i = 0; i < v; i++) {
            adj[i] = new LinkedList<Integer>();
        }
    }

    public void addEdge(int s, int t) {
        adj[s].add(t);
        adj[t].add(s);
    }

    private void dfs(int s, int t) {
        if (s == t) return;
        found = false;
        boolean[] visited = new boolean[v];
        int[] prev = new int[v];
        for (int i = 0; i < v; i++) {
            prev[i] = -1;
        }
        recurDfs(s, t, visited, prev);
        print(prev, s, t);
    }

    private void recurDfs(int w, int t, boolean[] visited, int[] prev) {
        if (found == true) return;
        visited[w] = true;
        if (w == t) {
            found = true;
            return;
        }

        for (int i = 0; i < adj[w].size(); i++) {
            int q = adj[w].get(i);
            if (!visited[q]) {
                prev[q] = w;
                recurDfs(q, t, visited, prev);
            }
        }

    }

    private void bfs(int s, int t) {
        if (s == t) return;
        boolean[] visited = new boolean[v];
        visited[s] = true;
        int[] prev = new int[v];
        for (int i = 0; i < v; i++) {
            prev[i] = -1;
        }
        LinkedList<Integer> queue = new LinkedList<Integer>();
        queue.offer(s);
        while (queue.size() > 0) {
            int w = queue.poll();
            for (int i = 0; i < adj[w].size(); i++) {
                int q = adj[w].get(i);
                if (!visited[q]) {
                    prev[q] = w;
                    if (q == t) {
                        print(prev, s, t);
                    }
                    visited[q] = true;
                    queue.offer(q);
                }
            }
        }
    }

    private void print(int[] prev, int s, int t) {
        if (prev[t] != -1 && s != t) {
            print(prev, s, prev[t]);
        }
        System.out.println(t + " ");
    }

    public static void main(String[] args) {
        Graph g = new Graph(8);
        g.addEdge(0, 1);
        g.addEdge(0, 3);
        g.addEdge(1, 2);
        g.addEdge(1, 4);
        g.addEdge(2, 5);
        g.addEdge(3, 4);
        g.addEdge(4, 5);
        g.addEdge(4, 6);
        g.addEdge(5, 7);
        g.addEdge(6, 7);
        g.bfs(0, 6);
        System.out.println("");
        g.dfs(0, 6);
    }
}