Add BFS algorithm

algorithms/bfs/CMakeLists.txt

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+target_sources(${PROJECT_NAME} PUBLIC test_bfs.cpp bfs.cpp)

algorithms/bfs/bfs.cpp

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+#include "bfs.hpp"
+
+std::vector<int64_t> bfs(
+    const std::vector<std::vector<int64_t>> &graph,
+    int64_t source_node
+) {
+    std::size_t n = graph.size();
+    std::vector<int64_t> distance(n, -1);
+    distance[source_node] = 0;
+    std::deque<int64_t> queue = {source_node};
+    while (false == queue.empty()) {
+        int64_t u = queue.front();
+        queue.pop_front();
+        for (int64_t v : graph[u]) {
+            if (-1 == distance[v]) {
+                distance[v] = distance[u] + 1;
+                queue.push_back(v);
+            }
+        }
+    }
+    return distance;
+}

algorithms/bfs/bfs.hpp

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+#include <deque>
+#include <vector>
+#include <cstdint>
+
+std::vector<int64_t> bfs(
+    const std::vector<std::vector<int64_t>> &graph,
+    int64_t source_node
+);

algorithms/bfs/test_bfs.cpp

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+#include <gtest/gtest.h>
+
+#include "bfs.hpp"
+
+TEST(BFS, Test0) {
+    std::vector<std::vector<int64_t>> graph = {
+        {1, 2},
+        {0, 2},
+        {0, 1},
+    };
+
+    int64_t source_node = 0;
+
+    std::vector<int64_t> distance = bfs(graph, 0);
+
+    std::vector<int64_t> expected_distance = {0, 1, 1};
+
+    ASSERT_EQ(distance, expected_distance);
+}
+
+TEST(BFS, Test1) {
+    std::vector<std::vector<int64_t>> graph = {
+        {1},
+        {2},
+        {},
+    };
+
+    int64_t source_node = 0;
+
+    std::vector<int64_t> distance = bfs(graph, 0);
+
+    std::vector<int64_t> expected_distance = {0, 1, 2};
+
+    ASSERT_EQ(distance, expected_distance);
+}
+
+TEST(BFS, Test2) {
+    std::vector<std::vector<int64_t>> graph = {
+        {1, 2},
+        {0, 2},
+        {0, 1},
+        {4},
+        {3},
+    };
+
+    int64_t source_node = 0;
+
+    std::vector<int64_t> distance = bfs(graph, 0);
+
+    std::vector<int64_t> expected_distance = {0, 1, 1, -1, -1};
+
+    ASSERT_EQ(distance, expected_distance);
+}