Merge pull request #406 from Lisanaaa/master

Thanks for your efort.

Data Structures/Trees/TreeTraversal.java

@@ -39,7 +39,7 @@ public class TreeTraversal {
 
 /**
 * The Node class which initializes a Node of a tree
-* Consists of all 3 traversal methods: printInOrder, printPostOrder & printPreOrder
+* Consists of all 4 traversal methods: printInOrder, printPostOrder， printPreOrder & printLevelOrder
 * printInOrder: LEFT -> ROOT -> RIGHT
 * printPreOrder: ROOT -> LEFT -> RIGHT
 * printPostOrder: LEFT -> RIGHT -> ROOT

Others/TopKWords.java

@@ -0,0 +1,84 @@
+import java.io.*;
+import java.util.*;
+
+/* display the most frequent K words in the file and the times it appear
+    in the file – shown in order (ignore case and periods) */
+
+public class TopKWords {
+    static class CountWords {
+        private String fileName;
+
+        public CountWords(String fileName) {
+            this.fileName = fileName;
+        }
+
+        public Map<String, Integer> getDictionary() {
+            Map<String, Integer> dictionary = new HashMap<>();
+            FileInputStream fis = null;
+
+            try {
+
+                fis = new FileInputStream(fileName);  // open the file
+                int in = 0;
+                String s = "";  // init a empty word
+                in = fis.read();  // read one character
+
+                while (-1 != in) {
+                    if (Character.isLetter((char)in)) {
+                        s += (char)in;  //if get a letter, append to s
+                    } else {
+                        // this branch means an entire word has just been read
+                        if (s.length() > 0) {
+                            // see whether word exists or not
+                            if (dictionary.containsKey(s)) {
+                                // if exist, count++
+                                dictionary.put(s, dictionary.get(s) + 1);
+                            } else {
+                                // if not exist, initiate count of this word with 1
+                                dictionary.put(s, 1);
+                            }
+                        }
+                        s = ""; // reInit a empty word
+                    }
+                    in = fis.read();
+                }
+                return dictionary;
+            } catch (IOException e) {
+                e.printStackTrace();
+            } finally {
+                try {
+                    // you always have to close the I/O streams
+                    fis.close();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
+            }
+            return null;
+        }
+    }
+    public static void main(String[] args) {
+        // you can replace the filePath with yours
+        CountWords cw = new CountWords("/Users/lisanaaa/Desktop/words.txt");
+        Map<String, Integer> dictionary = cw.getDictionary();  // get the words dictionary: {word: frequency}
+
+        // we change the map to list for convenient sort
+        List<Map.Entry<String, Integer>> list = new ArrayList<>(dictionary.entrySet());
+
+        // sort by lambda valueComparator
+        list.sort(Comparator.comparing(
+                m -> m.getValue())
+        );
+
+        Scanner input = new Scanner(System.in);
+        int k = input.nextInt();
+        while (k > list.size()) {
+            System.out.println("Retype a number, your number is too large");
+            input = new Scanner(System.in);
+            k = input.nextInt();
+        }
+        for (int i = 0; i < k; i++) {
+            System.out.println(list.get(list.size() - i - 1));
+        }
+    }
+}
+