Method dfs

java/30_graph/Graph.java:91–100

(int s, int t)

    boolean found = false; // 全局变量或者类成员变量

    public void dfs(int s, int t) {
        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;