对于分类结构可以用树形来形象地表示。比如：文件系统就是典型的例子。
树中的结点具有父子关系。我们在显示的时候，把子项向右缩进（用空格，不是tab），并添加必要的连接线，以使其层次关系更醒目。
下面的代码就是为了这个目的的，请仔细阅读源码，并填写划线部分缺少的代码。
import java.util.*;class MyTree
{ private Map<String, List<String>> map_ch = new HashMap<String, List<String>>(); private Map<String,String> map_pa = new HashMap<String,String>(); public void add(String parent, String child) { map_pa.put(child, parent); List<String> lst = map_ch.get(parent); if(lst==null){ lst = new ArrayList<String>(); map_ch.put(parent, lst); } lst.add(child); } public String get_parent(String me){ return map_pa.get(me); } public List<String> get_child(String me){ return map_ch.get(me); } private String space(int n) { String s = ""; for(int i=0; i<n; i++) s += ' '; return s; } private boolean last_child(String x){ String pa = map_pa.get(x); if(pa==null) return true; List<String> lst = map_ch.get(pa); return lst.get(lst.size()-1).equals(x); } public void show(String x){ String s = "+--" + x; String pa = x; while(true){ pa = map_pa.get(pa); if(pa==null) break; s = ___________________________________ ; // 填空 } System.out.println(s); } public void dfs(String x){ show(x); List<String> lst = map_ch.get(x); if(lst==null) return; for(String it: lst){ dfs(it); } }}public class TreeView
{ public static void main(String[] args) { MyTree tree = new MyTree(); tree.add("root", "dog"); tree.add("root", "cat"); tree.add("root", "duck"); tree.add("dog", "AAdog"); tree.add("dog", "BBdog"); tree.add("dog", "CCdog"); tree.add("AAdog", "AAdog01"); tree.add("AAdog", "AAdog02"); tree.add("cat", "XXcat"); tree.add("cat", "YYcat"); tree.add("XXcat","XXcat-oo"); tree.add("XXcat","XXcat-qq"); tree.add("XXcat-qq", "XXcat-qq-hahah"); tree.add("duck", "TTduck"); tree.add("TTduck", "TTduck-001"); tree.add("TTduck", "TTduck-002"); tree.add("TTduck", "TTduck-003"); tree.add("YYcat","YYcat.hello"); tree.add("YYcat","YYcat.yes"); tree.add("YYcat","YYcat.me"); tree.dfs("root"); }}对于题目中的测试数据，输出结果:
+--root +--dog | +--AAdog | | +--AAdog01 | | +--AAdog02 | +--BBdog | +--CCdog +--cat | +--XXcat | | +--XXcat-oo | | +--XXcat-qq | | +--XXcat-qq-hahah | +--YYcat | +--YYcat.hello | +--YYcat.yes | +--YYcat.me +--duck +--TTduck +--TTduck-001 +--TTduck-002 +--TTduck-003如有平字体对齐问题，可以参见图【p1.png】注意，只填写划线部分缺少的代码，不要抄写已有的代码或符号。
package 树形显示;import java.util.*;class MyTree{ private Map > map_ch = new HashMap >(); private Map map_pa = new HashMap (); public void add(String parent, String child) { map_pa.put(child, parent); List lst = map_ch.get(parent); if(lst==null){ lst = new ArrayList (); map_ch.put(parent, lst); } lst.add(child); } public String get_parent(String me){ return map_pa.get(me); } public List get_child(String me){ return map_ch.get(me); } private String space(int n) { String s = ""; for(int i=0; i lst = map_ch.get(pa); return lst.get(lst.size()-1).equals(x); } public void show(String x){ String s = "+--" + x; String pa = x; while(true){ pa = map_pa.get(pa); if(pa==null) break; s = !last_child(pa) ? "|"+space(5)+s : space(5)+s; // 填空 } System.out.println(s); } public void dfs(String x){ show(x); List lst = map_ch.get(x); if(lst==null) return; for(String it: lst){ dfs(it); } }}public class Main{ public static void main(String[] args) { MyTree tree = new MyTree(); tree.add("root", "dog"); tree.add("root", "cat"); tree.add("root", "duck"); tree.add("dog", "AAdog"); tree.add("dog", "BBdog"); tree.add("dog", "CCdog"); tree.add("AAdog", "AAdog01"); tree.add("AAdog", "AAdog02"); tree.add("cat", "XXcat"); tree.add("cat", "YYcat"); tree.add("XXcat","XXcat-oo"); tree.add("XXcat","XXcat-qq"); tree.add("XXcat-qq", "XXcat-qq-hahah"); tree.add("duck", "TTduck"); tree.add("TTduck", "TTduck-001"); tree.add("TTduck", "TTduck-002"); tree.add("TTduck", "TTduck-003"); tree.add("YYcat","YYcat.hello"); tree.add("YYcat","YYcat.yes"); tree.add("YYcat","YYcat.me"); tree.dfs("root"); }}