From a9312b3901fd3237dc2d0dfe6230f026f04b9dc2 Mon Sep 17 00:00:00 2001
From: Tushar Mohan <tushar.mohan2001@gmail.com>
Date: Thu, 28 Oct 2021 07:18:58 +0530
Subject: [PATCH] feat: a different implementation of checking bipartite-ness
 of a graph (#1769)

* feat: a different implementation of checking bipartite-ness of a graph

* updating DIRECTORY.md

* fix: code formatter error

* fix: requested changes

* fix: request changes

* fix : requested changed

* pass parameters by reference

* pass parameters by reference

* fix : visited to pointer

* fix : line length below 80 chars

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <halfpacho@gmail.com>
Co-authored-by: Ayaan Khan <ayaankhan98@gmail.com>
---
 DIRECTORY.md                  |   1 +
 graph/is_graph_bipartite2.cpp | 134 ++++++++++++++++++++++++++++++++++
 2 files changed, 135 insertions(+)
 create mode 100644 graph/is_graph_bipartite2.cpp

diff --git a/DIRECTORY.md b/DIRECTORY.md
index ae84537c9..618142c55 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -124,6 +124,7 @@
   * [Hamiltons Cycle](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/graph/hamiltons_cycle.cpp)
   * [Hopcroft Karp](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/graph/hopcroft_karp.cpp)
   * [Is Graph Bipartite](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/graph/is_graph_bipartite.cpp)
+  * [Is Graph Bipartite2](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/graph/is_graph_bipartite2.cpp)
   * [Kosaraju](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/graph/kosaraju.cpp)
   * [Kruskal](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/graph/kruskal.cpp)
   * [Lowest Common Ancestor](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/graph/lowest_common_ancestor.cpp)
diff --git a/graph/is_graph_bipartite2.cpp b/graph/is_graph_bipartite2.cpp
new file mode 100644
index 000000000..07374da9a
--- /dev/null
+++ b/graph/is_graph_bipartite2.cpp
@@ -0,0 +1,134 @@
+/**
+ * @brief Check whether a given graph is bipartite or not
+ * @details
+ * A bipartite graph is the one whose nodes can be divided into two 
+ * disjoint sets in such a way that the nodes in a set are not 
+ * connected to each other at all, i.e. no intra-set connections. 
+ * The only connections that exist are that of inter-set, 
+ * i.e. the nodes from one set are connected to a subset of nodes 
+ * in the other set.
+ * In this implementation, using a graph in the form of adjacency 
+ * list, check whether the given graph is a bipartite or not.
+ * 
+ * References used: [GeeksForGeeks](https://www.geeksforgeeks.org/bipartite-graph/)
+ * @author [tushar2407](https://github.com/tushar2407)
+ */
+#include <iostream> /// for IO operations
+#include <queue>    /// for queue data structure
+#include <vector>   /// for vector data structure
+#include <cassert>  /// for assert
+
+/**
+ * @namespace graph
+ * @brief Graphical algorithms
+ */
+namespace graph {
+/**
+ * @brief function to check whether the passed graph is bipartite or not
+ * @param graph is a 2D matrix whose rows or the first index signify the node
+ * and values in that row signify the nodes it is connected to
+ * @param index is the valus of the node currently under observation
+ * @param visited is the vector which stores whether a given node has been 
+ * traversed or not yet
+ * @returns boolean
+ */
+bool checkBipartite(
+    const std::vector<std::vector<int64_t>> &graph, 
+    int64_t index, 
+    std::vector<int64_t> *visited
+)
+{
+    std::queue<int64_t> q; ///< stores the neighbouring node indexes in squence 
+                           /// of being reached
+    q.push(index);         /// insert the current node into the queue
+    (*visited)[index] = 1; /// mark the current node as travelled
+    while(q.size())
+    {
+        int64_t u = q.front();
+        q.pop();
+        for(uint64_t i=0;i<graph[u].size();i++)
+        {
+            int64_t v = graph[u][i];    ///< stores the neighbour of the current node
+            if(!(*visited)[v])          /// check whether the neighbour node is 
+                                        /// travelled already or not
+            {
+                (*visited)[v] = ((*visited)[u]==1)?-1:1;  /// colour the neighbouring node with 
+                                                          /// different colour than the current node
+                q.push(v);      /// insert the neighbouring node into the queue
+            }
+            else if((*visited)[v] == (*visited)[u])   /// if both the current node and its neighbour
+                                                      /// has the same state then it is not a bipartite graph
+            {
+                return false;
+            }
+        }
+    }
+    return true;    /// return true when all the connected nodes of the current 
+                    /// nodes are travelled and satisify all the above conditions
+}
+/**
+ * @brief returns true if the given graph is bipartite else returns false
+ * @param graph is a 2D matrix whose rows or the first index signify the node 
+ * and values in that row signify the nodes it is connected to
+ * @returns booleans
+ */
+bool isBipartite(const std::vector<std::vector<int64_t>> &graph) 
+{
+    std::vector<int64_t> visited(graph.size()); ///< stores boolean values 
+                                                /// which signify whether that node had been visited or not
+    
+    for(uint64_t i=0;i<graph.size();i++)
+    {    
+        if(!visited[i]) /// if the current node is not visited then check 
+                        /// whether the sub-graph of that node is a bipartite or not
+        {
+            if(!checkBipartite(graph, i, &visited))
+            {
+                return false;
+            }
+        }
+    }
+    return true;
+}
+}  // namespace graph
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test()
+{
+    std::vector<std::vector<int64_t>> graph = {
+        {1,3},
+        {0,2},
+        {1,3},
+        {0,2}
+    };
+
+    assert(graph::isBipartite(graph) == true); /// check whether the above 
+                                               /// defined graph is indeed bipartite
+
+    std::vector<std::vector<int64_t>> graph_not_bipartite = {
+        {1,2,3},
+        {0,2},
+        {0,1,3},
+        {0,2}
+    };
+
+    assert(graph::isBipartite(graph_not_bipartite) == false); /// check whether 
+                                                              /// the above defined graph is indeed bipartite
+    std::cout << "All tests have successfully passed!\n";
+}
+/**
+ * @brief Main function
+ * Instantitates a dummy graph of a small size with 
+ * a few edges between random nodes.
+ * On applying the algorithm, it checks if the instantiated 
+ * graph is bipartite or not.
+ * @returns 0 on exit
+ */
+int main()
+{
+    test();  // run self-test implementations
+    return 0;
+}