chore: import upstream snapshot with attribution

graph/depth_first_search_with_stack.cpp

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+/**
+ *
+ * @file
+ * @brief [Depth First Search Algorithm using Stack
+ * (Depth First Search Algorithm)](https://en.wikipedia.org/wiki/Depth-first_search)
+ *
+ * @author [Ayaan Khan](http://github.com/ayaankhan98)
+ * @author [Saurav Uppoor](https://github.com/sauravUppoor)
+ *
+ * @details
+ * Depth First Search also quoted as DFS is a Graph Traversal Algorithm.
+ * Time Complexity O(|V| + |E|) where V is number of vertices and E
+ * is number of edges in graph.
+ *
+ * Application of Depth First Search are
+ *
+ * 1. Finding connected components
+ * 2. Finding 2-(edge or vertex)-connected components.
+ * 3. Finding 3-(edge or vertex)-connected components.
+ * 4. Finding the bridges of a graph.
+ * 5. Generating words in order to plot the limit set of a group.
+ * 6. Finding strongly connected components.
+ *
+ * <h4>Working</h4>
+ * 1. Mark all vertices as unvisited (colour it WHITE).
+ * 2. Push starting vertex into the stack and colour it GREY.
+ * 3. Once a node is popped out of the stack and is coloured GREY, we colour it BLACK.
+ * 4. Push all its neighbours which are not coloured BLACK.
+ * 5. Repeat steps 4 and 5 until the stack is empty.
+ */
+
+#include <iostream> 	 /// for IO operations
+#include <stack>    	 /// header for std::stack
+#include <vector>   	 /// header for std::vector
+#include <cassert>  	 /// header for preprocessor macro assert() 
+#include <limits>   	 /// header for limits of integral types
+
+constexpr int WHITE = 0; /// indicates the node hasn't been explored
+constexpr int GREY = 1;	 /// indicates node is in stack waiting to be explored
+constexpr int BLACK = 2; /// indicates node has already been explored
+constexpr int64_t INF = std::numeric_limits<int16_t>::max();
+
+
+/**
+ * @namespace graph
+ * @brief Graph algorithms
+ */
+namespace graph {
+/**
+ * @namespace depth_first_search
+ * @brief Functions for [Depth First Search](https://en.wikipedia.org/wiki/Depth-first_search) algorithm
+ */
+namespace depth_first_search {
+/**
+ * @brief
+ * Adds and edge between two vertices of graph say u and v in this
+ * case.
+ *
+ * @param adj Adjacency list representation of graph
+ * @param u first vertex
+ * @param v second vertex
+ *
+ */
+void addEdge(std::vector<std::vector<size_t>> *adj, size_t u, size_t v) {
+	/*
+	*
+	* Here we are considering undirected graph that's the
+	* reason we are adding v to the adjacency list representation of u
+	* and also adding u to the adjacency list representation of v
+	*
+	*/
+	(*adj)[u - 1].push_back(v - 1);
+}
+
+/**
+ *
+ * @brief
+ * Explores the given vertex, exploring a vertex means traversing
+ * over all the vertices which are connected to the vertex that is
+ * currently being explored and push it onto the stack.
+ *
+ * @param adj graph
+ * @param start starting vertex for DFS
+ * @return vector with nodes stored in the order of DFS traversal
+ *
+ */
+std::vector<size_t> dfs(const std::vector<std::vector<size_t> > &graph, size_t start) {
+    /// checked[i] stores the status of each node
+    std::vector<size_t> checked(graph.size(), WHITE), traversed_path;
+
+    checked[start] = GREY;
+    std::stack<size_t> stack;
+    stack.push(start);
+
+    /// while stack is not empty we keep exploring the node on top of stack
+    while (!stack.empty()) {
+        int act = stack.top();
+        stack.pop();
+
+        if (checked[act] == GREY) {
+            /// push the node to the final result vector
+            traversed_path.push_back(act + 1);
+
+            /// exploring the neighbours of the current node
+            for (auto it : graph[act]) {
+                stack.push(it);
+                if (checked[it] != BLACK) {
+                    checked[it] = GREY;
+                }
+            }
+            checked[act] = BLACK;  /// Node has been explored
+        }
+    }
+    return traversed_path;
+}
+}  // namespace depth_first_search
+}  // namespace graph
+
+/**
+ * Self-test implementations
+ * @returns none
+ */
+static void tests() {
+	size_t start_pos;
+
+	/// Test 1
+	std::cout << "Case 1: " << std::endl;
+	start_pos = 1;
+	std::vector<std::vector<size_t> > g1(3, std::vector<size_t>());
+
+	graph::depth_first_search::addEdge(&g1, 1, 2);
+	graph::depth_first_search::addEdge(&g1, 2, 3);
+	graph::depth_first_search::addEdge(&g1, 3, 1);
+
+	std::vector<size_t> expected1 {1, 2, 3}; /// for the above sample data, this is the expected output
+	assert(graph::depth_first_search::dfs(g1, start_pos - 1) == expected1);
+	std::cout << "Passed" << std::endl;
+
+	/// Test 2
+	std::cout << "Case 2: " << std::endl;
+	start_pos = 1;
+	std::vector<std::vector<size_t> > g2(4, std::vector<size_t>());
+
+	graph::depth_first_search::addEdge(&g2, 1, 2);
+	graph::depth_first_search::addEdge(&g2, 1, 3);
+	graph::depth_first_search::addEdge(&g2, 2, 4);
+	graph::depth_first_search::addEdge(&g2, 4, 1);
+
+	std::vector<size_t> expected2 {1, 3, 2, 4}; /// for the above sample data, this is the expected output
+	assert(graph::depth_first_search::dfs(g2, start_pos - 1) == expected2);
+	std::cout << "Passed" << std::endl;
+
+	/// Test 3
+	std::cout << "Case 3: " << std::endl;
+	start_pos = 2;
+	std::vector<std::vector<size_t> > g3(4, std::vector<size_t>());
+
+	graph::depth_first_search::addEdge(&g3, 1, 2);
+	graph::depth_first_search::addEdge(&g3, 1, 3);
+	graph::depth_first_search::addEdge(&g3, 2, 4);
+	graph::depth_first_search::addEdge(&g3, 4, 1);
+
+	std::vector<size_t> expected3 {2, 4, 1, 3}; /// for the above sample data, this is the expected output
+	assert(graph::depth_first_search::dfs(g3, start_pos - 1) == expected3);
+	std::cout << "Passed" << std::endl;
+
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    tests();  // execute the tests
+
+    size_t vertices = 0, edges = 0, start_pos = 1;
+	std::vector<size_t> traversal;
+
+    std::cout << "Enter the Vertices : ";
+	std::cin >> vertices;
+	std::cout << "Enter the Edges : ";
+	std::cin >> edges;
+
+    /// creating a graph
+    std::vector<std::vector<size_t> > adj(vertices, std::vector<size_t>());
+
+    /// taking input for the edges
+    std::cout << "Enter the vertices which have edges between them : " << std::endl;
+	while (edges--) {
+		size_t u = 0, v = 0;
+		std::cin >> u >> v;
+		graph::depth_first_search::addEdge(&adj, u, v);
+	}
+
+    /// taking input for the starting position
+    std::cout << "Enter the starting vertex [1,n]: " << std::endl;
+	std::cin >> start_pos;
+	start_pos -= 1;
+	traversal = graph::depth_first_search::dfs(adj, start_pos);
+
+    /// Printing the order of traversal
+    for (auto x : traversal) {
+		std::cout << x << ' ';
+	}
+
+    return 0;
+}