Bug fixes and improvements (#1318)

* Sync zh and zh-hant versions

* Update en/README.md

* Add a Q&A for chapter of introduction

* Update the callout headers

* Sync zh ang zh-hant versions

* Bug fixes

zh-hant/codes/java/chapter_graph/graph_dfs.java

@@ -28,7 +28,7 @@ public class graph_dfs {
     static List<Vertex> graphDFS(GraphAdjList graph, Vertex startVet) {
         // 頂點走訪序列
         List<Vertex> res = new ArrayList<>();
-        // 雜湊表，用於記錄已被訪問過的頂點
+        // 雜湊集合，用於記錄已被訪問過的頂點
         Set<Vertex> visited = new HashSet<>();
         dfs(graph, visited, res, startVet);
         return res;