From ce3ce27588c4bd8c6fcc8a7867e843f1199ad768 Mon Sep 17 00:00:00 2001
From: fadli0029 <mfarsani@ucsd.edu>
Date: Fri, 12 Jul 2024 22:30:36 -0700
Subject: [PATCH] [ADD] bellman ford

---
 algorithms/CMakeLists.txt         |   6 ++
 algorithms/graph/bellman_ford.cpp | 129 +++++++++++++++++++++++++++++-
 algorithms/graph/dijkstra.cpp     |   1 +
 3 files changed, 132 insertions(+), 4 deletions(-)

diff --git a/algorithms/CMakeLists.txt b/algorithms/CMakeLists.txt
index 8930761..fbc401c 100644
--- a/algorithms/CMakeLists.txt
+++ b/algorithms/CMakeLists.txt
@@ -26,3 +26,9 @@ add_executable(dijkstra_exe graph/dijkstra.cpp)
 target_include_directories(dijkstra_exe PUBLIC ${Catch2_INCLUDE_DIRS})
 target_link_libraries(dijkstra_exe Catch2::Catch2WithMain)
 add_test(NAME DijkstraTest COMMAND dijkstra_exe)
+
+# Add bellman-ford
+add_executable(bellman_ford_exe graph/bellman_ford.cpp)
+target_include_directories(bellman_ford_exe PUBLIC ${Catch2_INCLUDE_DIRS})
+target_link_libraries(bellman_ford_exe Catch2::Catch2WithMain)
+add_test(NAME BellmanFordTest COMMAND bellman_ford_exe)
diff --git a/algorithms/graph/bellman_ford.cpp b/algorithms/graph/bellman_ford.cpp
index a6fe24a..f3cf382 100644
--- a/algorithms/graph/bellman_ford.cpp
+++ b/algorithms/graph/bellman_ford.cpp
@@ -1,4 +1,125 @@
-// There's two variant:
-// 1. We want shortest path, so do N-1 iterations.
-// 2. A constraint says at most k edges are allowed to get the shortest distance between two nodes, so do k iterations.
-//    (see letcode graph explore card bellman ford chapter).
+/* =====================================================================================
+   This file contains the implementation of the Bellman-Ford's algorithm. It returns
+   the shortest distance from the starting node to all other nodes.
+
+   @param n:            the number of vertices in the graph
+   @param e:            the edge list representation of the graph
+   @param s:            the starting node
+   @return vector<int>: the shortest distance from the starting node to all other nodes,
+                        {-1} if there is a negative cycle in the graph
+
+   @author: Muhammad Fadli Alim Arsani
+======================================================================================*/
+
+#define CATCH_CONFIG_MAIN
+#include <catch2/catch_test_macros.hpp>
+
+#include "bits/stdc++.h"
+using namespace std;
+
+vector<int> bellman_ford(int n, vector<vector<int>>& e, int s) {
+  vector<int> dists(n, INT_MAX);
+  dists[s] = 0;
+
+  for (int i=0; i<n-1; i++) {
+    for (const auto& edge : e) {
+      int u = edge[0], v = edge[1], w = edge[2];
+
+      // Edge relaxation step
+      if (dists[u] != INT_MAX && dists[u] + w < dists[v]) {
+        dists[v] = dists[u] + w;
+      }
+    }
+  }
+
+  // Check for negative cycle existence by doing
+  // one more iteration (i.e: n iteration instead of n-1)
+  for (const auto& edge : e) {
+    int u = edge[0], v = edge[1], w = edge[2];
+
+    // Edge relaxation step
+    if (dists[u] != INT_MAX && dists[u] + w < dists[v]) {
+      return {-1};
+    }
+  }
+
+  return dists;
+}
+
+TEST_CASE("Bellman-Ford algorithm test cases") {
+    SECTION("Simple graph without negative weights") {
+        int n = 5;
+        vector<vector<int>> e = {
+            {0, 1, 2},
+            {0, 3, 6},
+            {1, 2, 3},
+            {1, 3, 8},
+            {1, 4, 5},
+            {2, 4, 7},
+            {3, 4, 9}
+        };
+        int start = 0;
+        vector<int> expected = {0, 2, 5, 6, 7};
+        vector<int> result = bellman_ford(n, e, start);
+
+        REQUIRE(result == expected);
+    }
+
+    SECTION("Graph with negative weights but no negative cycle") {
+        int n = 5;
+        vector<vector<int>> e = {
+            {0, 1, -1},
+            {0, 2, 4},
+            {1, 2, 3},
+            {1, 3, 2},
+            {1, 4, 2},
+            {3, 2, 5},
+            {3, 1, 1},
+            {4, 3, -3}
+        };
+        int start = 0;
+        vector<int> expected = {0, -1, 2, -2, 1};
+        vector<int> result = bellman_ford(n, e, start);
+
+        REQUIRE(result == expected);
+    }
+
+    SECTION("Graph with a negative cycle") {
+        int n = 4;
+        vector<vector<int>> e = {
+            {0, 1, 1},
+            {1, 2, -1},
+            {2, 3, -1},
+            {3, 0, -1}
+        };
+        int start = 0;
+        vector<int> expected = {-1};
+        vector<int> result = bellman_ford(n, e, start);
+
+        REQUIRE(result == expected);
+    }
+
+    SECTION("Disconnected graph") {
+        int n = 4;
+        vector<vector<int>> e = {
+            {0, 1, 4},
+            {1, 2, 5}
+        };
+        int start = 0;
+        vector<int> expected = {0, 4, 9, INT_MAX};
+        vector<int> result = bellman_ford(n, e, start);
+
+        REQUIRE(result == expected);
+    }
+
+    SECTION("Graph with a single node") {
+        int n = 1;
+        vector<vector<int>> e = {};
+        int start = 0;
+        vector<int> expected = {0};
+        vector<int> result = bellman_ford(n, e, start);
+
+        REQUIRE(result == expected);
+    }
+}
+
diff --git a/algorithms/graph/dijkstra.cpp b/algorithms/graph/dijkstra.cpp
index b070a92..d8f72f9 100644
--- a/algorithms/graph/dijkstra.cpp
+++ b/algorithms/graph/dijkstra.cpp
@@ -5,6 +5,7 @@
    @param n: the number of vertices in the graph
    @param g: the adjacency list representation of the graph
    @param s: the starting node
+   @return vector<int>: the shortest distance from the starting node to all other nodes
 
    @author: Muhammad Fadli Alim Arsani
 =====================================================================================*/