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package class_2022_08_3_week;
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import java.util.ArrayList;
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// 给你一个 n 个节点的 有向图 ，节点编号为 0 到 n - 1 ，其中每个节点 至多 有一条出边。
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// 图用一个大小为 n 下标从 0 开始的数组 edges 表示，
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// 节点 i 到节点 edges[i] 之间有一条有向边。如果节点 i 没有出边，那么 edges[i] == -1 。
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// 请你返回图中的 最长 环，如果没有任何环，请返回 -1 。
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// 一个环指的是起点和终点是 同一个 节点的路径。
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// 用强联通分量
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// 测试链接 : https://leetcode.cn/problems/longest-cycle-in-a-graph/
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public class Code01_LongestCycleInGraph1 {
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public static int longestCycle(int[] edges) {
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int n = edges.length;
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ArrayList<ArrayList<Integer>> graph = new ArrayList<ArrayList<Integer>>();
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for (int i = 0; i < n; i++) {
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graph.add(new ArrayList<Integer>());
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}
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for (int i = 0; i < n; i++) {
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if (edges[i] != -1) {
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graph.get(i).add(edges[i]);
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}
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}
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StronglyConnectedComponents connectedComponents = new StronglyConnectedComponents(graph);
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int m = connectedComponents.getSccn() + 1;
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int[] cnt = new int[m];
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int[] scc = connectedComponents.getScc();
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for (int i = 0; i < n; i++) {
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cnt[scc[i]]++;
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}
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int ans = -1;
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for (int count : cnt) {
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ans = Math.max(ans, count > 1 ? count : -1);
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}
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return ans;
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}
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public static class StronglyConnectedComponents {
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public ArrayList<ArrayList<Integer>> nexts;
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public int n;
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public int stackSize;
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public int cnt;
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public int sccn;
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public int[] stack;
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public int[] dfn;
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public int[] low;
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public int[] scc;
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public StronglyConnectedComponents(ArrayList<ArrayList<Integer>> edges) {
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nexts = edges;
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init();
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scc();
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}
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private void init() {
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n = nexts.size();
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stackSize = 0;
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cnt = 0;
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sccn = 0;
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stack = new int[n];
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dfn = new int[n];
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low = new int[n];
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scc = new int[n];
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}
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private void scc() {
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for (int i = 0; i < n; i++) {
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if (dfn[i] == 0) {
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tarjan(i);
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}
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}
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}
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private void tarjan(int p) {
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low[p] = dfn[p] = ++cnt;
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stack[stackSize++] = p;
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for (int q : nexts.get(p)) {
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if (dfn[q] == 0) {
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tarjan(q);
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}
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if (scc[q] == 0) {
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low[p] = Math.min(low[p], low[q]);
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}
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}
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if (low[p] == dfn[p]) {
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sccn++;
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int top = 0;
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do {
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top = stack[--stackSize];
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scc[top] = sccn;
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} while (top != p);
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}
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}
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public int[] getScc() {
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return scc;
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}
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public int getSccn() {
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return sccn;
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}
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}
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}
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