diff --git a/graph/number_of_paths.cpp b/graph/number_of_paths.cpp new file mode 100644 index 00000000000..45774021404 --- /dev/null +++ b/graph/number_of_paths.cpp @@ -0,0 +1,151 @@ +/** + * @file + * @brief Algorithm to count paths between two nodes in a directed graph using DFS + * @details + * This algorithm implements Depth First Search (DFS) to count the number of + * possible paths between two nodes in a directed graph. It is represented using + * an adjacency matrix. The algorithm recursively traverses the graph to find + * all paths from the source node `u` to the destination node `v`. + * + * @author [Aditya Borate](https://github.com/adi776borate) + * @see https://en.wikipedia.org/wiki/Path_(graph_theory) + */ + +#include /// for std::vector +#include /// for IO operations +#include /// for assert +#include /// for fixed-size integer types (e.g., std::uint32_t) + +/** + * @namespace graph + * @brief Graph algorithms + */ +namespace graph { + + /** + * @brief Helper function to perform DFS and count the number of paths from node `u` to node `v` + * @param A adjacency matrix representing the graph (1: edge exists, 0: no edge) + * @param u the starting node + * @param v the destination node + * @param n the number of nodes in the graph + * @param visited a vector to keep track of visited nodes in the current DFS path + * @returns the number of paths from node `u` to node `v` + */ + std::uint32_t count_paths_dfs(const std::vector>& A, + std::uint32_t u, + std::uint32_t v, + std::uint32_t n, + std::vector& visited) { + if (u == v) { + return 1; // Base case: Reached the destination node + } + + visited[u] = true; // Mark the current node as visited + std::uint32_t path_count = 0; // Count of all paths from `u` to `v` + + for (std::uint32_t i = 0; i < n; i++) { + if (A[u][i] == 1 && !visited[i]) { // Check if there is an edge and the node is not visited + path_count += count_paths_dfs(A, i, v, n, visited); // Recursively explore paths from `i` to `v` + } + } + + visited[u] = false; // Unmark the current node as visited (backtracking) + return path_count; + } + + + /** + * @brief Counts the number of paths from node `u` to node `v` in a directed graph + * using Depth First Search (DFS) + * + * @param A adjacency matrix representing the graph (1: edge exists, 0: no edge) + * @param u the starting node + * @param v the destination node + * @param n the number of nodes in the graph + * @returns the number of paths from node `u` to node `v` + */ + std::uint32_t count_paths(const std::vector>& A, + std::uint32_t u, + std::uint32_t v, + std::uint32_t n) { + // Check for invalid nodes or empty graph + if (u >= n || v >= n || A.empty() || A[0].empty()) { + return 0; // No valid paths if graph is empty or nodes are out of bounds + } + + std::vector visited(n, false); // Initialize a visited vector for tracking nodes + return count_paths_dfs(A, u, v, n, visited); // Start DFS + } + +} // namespace graph + +/** + * @brief Self-test implementations + * @returns void + */ +static void test() { + // Test case 1: Simple directed graph with multiple paths + std::vector> graph1 = { + {0, 1, 0, 1, 0}, + {0, 0, 1, 0, 1}, + {0, 0, 0, 0, 1}, + {0, 0, 1, 0, 0}, + {0, 0, 0, 0, 0} + }; + std::uint32_t n1 = 5, u1 = 0, v1 = 4; + assert(graph::count_paths(graph1, u1, v1, n1) == 3); // There are 3 paths from node 0 to 4 + + // Test case 2: No possible path (disconnected graph) + std::vector> graph2 = { + {0, 1, 0, 0, 0}, + {0, 0, 0, 0, 0}, + {0, 0, 0, 0, 1}, + {0, 0, 1, 0, 0}, + {0, 0, 0, 0, 0} + }; + std::uint32_t n2 = 5, u2 = 0, v2 = 4; + assert(graph::count_paths(graph2, u2, v2, n2) == 0); // No path from node 0 to 4 + + // Test case 3: Cyclic graph with multiple paths + std::vector> graph3 = { + {0, 1, 0, 0, 0}, + {0, 0, 1, 1, 0}, + {1, 0, 0, 0, 1}, + {0, 0, 1, 0, 1}, + {0, 0, 0, 0, 0} + }; + std::uint32_t n3 = 5, u3 = 0, v3 = 4; + assert(graph::count_paths(graph3, u3, v3, n3) == 3); // There are 3 paths from node 0 to 4 + + // Test case 4: Single node graph (self-loop) + std::vector> graph4 = { + {0} + }; + std::uint32_t n4 = 1, u4 = 0, v4 = 0; + assert(graph::count_paths(graph4, u4, v4, n4) == 1); // There is self-loop, so 1 path from node 0 to 0 + + // Test case 5: Empty graph (no nodes, no paths) + std::vector> graph5 = {{}}; + int n5 = 0, u5 = 0, v5 = 0; + assert(graph::count_paths(graph5, u5, v5, n5) == 0); // There are no paths in an empty graph + + // Test case 6: Invalid nodes (out of bounds) + std::vector> graph6 = { + {0, 1, 0}, + {0, 0, 1}, + {0, 0, 0} + }; + int n6 = 3, u6 = 0, v6 = 5; // Node `v` is out of bounds (n = 3, so valid indices are 0, 1, 2) + assert(graph::count_paths(graph6, u6, v6, n6) == 0); // Should return 0 because `v = 5` is invalid + + std::cout << "All tests have successfully passed!\n"; +} + +/** + * @brief Main function + * @returns 0 on exit + */ +int main() { + test(); // Run self-test implementations + return 0; +}