some programs

graph/ECNA2001_sort_letters.cpp

@@ -0,0 +1,10 @@
+#include <iostream>
+
+using namespace std;
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+    
+    return 0;
+}

graph/luogu_P3916_graph_iteration.cpp

@@ -0,0 +1,75 @@
+#include <cstring>
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+class Graph {
+public:
+    using VertexId = int;
+    using EdgeId = int;
+
+    struct Edge {
+        VertexId target_vertex = -1;
+        EdgeId next_edge = -1;
+    };
+
+    vector<Edge> edges;
+    vector<EdgeId> first_edges;
+
+    explicit Graph(int vertices_amount, int edges_amount = 0) : first_edges(vertices_amount, -1) {
+        edges.reserve(edges_amount);
+    }
+
+    void add_edge(VertexId source_vertex, VertexId target_vertex) {
+        const EdgeId edge_id = edges.size();
+        edges.push_back({.target_vertex = target_vertex, .next_edge = first_edges[source_vertex]}); // 所以遍历顺序相反！
+        first_edges[source_vertex] = edge_id;
+    }
+};
+
+constexpr int MAX_V = 1e5 + 5;
+
+int v, e;
+int ans_cache[MAX_V];
+
+int dfs(const Graph& graph, int vertex, int start_vertex) {
+    if (ans_cache[vertex] != -1) { return ans_cache[vertex]; }
+    ans_cache[vertex] = start_vertex;
+    for (int edge_id = graph.first_edges[vertex]; edge_id != -1; edge_id = graph.edges[edge_id].next_edge) {
+        dfs(graph, graph.edges[edge_id].target_vertex, start_vertex);
+    }
+    return ans_cache[vertex];
+}
+
+int dfs(const Graph& graph, int vertex) {
+    // 反向建图，从大到小 dfs
+    return dfs(graph, vertex, vertex);
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> v >> e;
+
+    Graph graph(v, e);
+
+    for (int _ = 0; _ < e; _++) {
+        int x, y;
+        cin >> x >> y;
+        graph.add_edge(y - 1, x - 1); // Reversed graph
+    }
+
+    memset(ans_cache, -1, sizeof(ans_cache));
+
+    for (int vertex = v - 1; vertex >= 0; vertex--) {
+        dfs(graph, vertex);
+    }
+    for (int vertex = 0; vertex < v; vertex++) {
+        cout << ans_cache[vertex] + 1 << " ";
+    }
+    cout << endl;
+
+    return 0;
+}

graph/luogu_P3916_graph_iteration_WRONG.cpp

@@ -0,0 +1,69 @@
+#include <cstring>
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+class Graph {
+public:
+    using VertexId = int;
+    using EdgeId = int;
+
+    struct Edge {
+        VertexId target_vertex = -1;
+        EdgeId next_edge = -1;
+    };
+
+    vector<Edge> edges;
+    vector<EdgeId> first_edges;
+
+    explicit Graph(int vertices_amount, int edges_amount = 0) : first_edges(vertices_amount, -1) {
+        edges.reserve(edges_amount);
+    }
+
+    void add_edge(VertexId source_vertex, VertexId target_vertex) {
+        const EdgeId edge_id = edges.size();
+        edges.push_back({.target_vertex = target_vertex, .next_edge = first_edges[source_vertex]}); // 所以遍历顺序相反！
+        first_edges[source_vertex] = edge_id;
+    }
+};
+
+constexpr int MAX_V = 1e5 + 5;
+
+int v, e;
+int ans_cache[MAX_V];
+
+int dfs(const Graph& graph, int vertex) {
+    // 如果从小到大 dfs，是不需要在此处取 max 的
+    if (ans_cache[vertex] != -1) { return ans_cache[vertex]; }
+    ans_cache[vertex] = vertex;
+    if (graph.first_edges[vertex] == -1) { return ans_cache[vertex]; }
+    for (int edge_id = graph.first_edges[vertex]; edge_id != -1; edge_id = graph.edges[edge_id].next_edge) {
+        ans_cache[vertex] = max(ans_cache[vertex], dfs(graph, graph.edges[edge_id].target_vertex));
+    }
+    return ans_cache[vertex];
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> v >> e;
+
+    Graph graph(v, e);
+
+    for (int _ = 0; _ < e; _++) {
+        int x, y;
+        cin >> x >> y;
+        graph.add_edge(x - 1, y - 1);
+    }
+
+    memset(ans_cache, -1, sizeof(ans_cache));
+
+    for (int vertex = 0; vertex < v; vertex++) {
+        cout << dfs(graph, vertex) + 1 << " ";
+    }
+    cout << endl;
+
+    return 0;
+}