add 606

Author: fishercoder1534

README.md

@@ -20,6 +20,7 @@ Your ideas/fixes/algorithms are more than welcome!
 
 |  #  |      Title     |   Solutions   | Time          | Space         | Difficulty  | Tag          | Notes
 |-----|----------------|---------------|---------------|---------------|-------------|--------------|-----
+|606|[Construct String from Binary Tree](https://leetcode.com/problems/construct-string-from-binary-tree/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_606.java) | O(n) |O(n) | Easy | Tree, Recursion
 |600|[Non-negative Integers without Consecutive Ones](https://leetcode.com/problems/non-negative-integers-without-consecutive-ones/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_600.java) | O(log2(max_int) = 32) | O(log2(max_int) = 32) | Hard | Bit Manipulation, DP
 |599|[Minimum Index Sum of Two Lists](https://leetcode.com/problems/minimum-index-sum-of-two-lists/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_599.java) | O(max(m,n))|O(max(m,n)) | Easy | HashMap
 |598|[Range Addition II](https://leetcode.com/problems/range-addition-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_598.java) | O(n) (n is the number of operations) |O(1) | Easy | 

src/main/java/com/fishercoder/solutions/_606.java

@@ -0,0 +1,53 @@
+package com.fishercoder.solutions;
+
+import com.fishercoder.common.classes.TreeNode;
+
+/**
+ * 606. Construct String from Binary Tree
+ *
+ * You need to construct a string consists of parenthesis and integers from a binary tree with the preorder traversing way.
+ The null node needs to be represented by empty parenthesis pair "()".
+ And you need to omit all the empty parenthesis pairs that don't affect the one-to-one mapping relationship between the string
+ and the original binary tree.
+
+ Example 1:
+ Input: Binary tree: [1,2,3,4]
+     1
+   /   \
+  2     3
+ /
+4
+
+ Output: "1(2(4))(3)"
+
+ Explanation: Originallay it needs to be "1(2(4)())(3()())",
+ but you need to omit all the unnecessary empty parenthesis pairs.
+ And it will be "1(2(4))(3)".
+
+ Example 2:
+ Input: Binary tree: [1,2,3,null,4]
+    1
+  /   \
+ 2     3
+  \
+  4
+
+ Output: "1(2()(4))(3)"
+
+ Explanation: Almost the same as the first example,
+ except we can't omit the first parenthesis pair to break the one-to-one mapping relationship between the input and the output.
+ */
+public class _606 {
+
+    public String tree2str(TreeNode t) {
+        if (t == null) return "";
+        String result = "" + t.val;
+        String left = tree2str(t.left);
+        String right = tree2str(t.right);
+        if (left.equals("") && right.equals("")) return result;
+        if (left.equals("")) return result + "()(" + right + ")";
+        if (right.equals("")) return result + "(" + left + ")";
+        return result + "(" + left + ")(" + right + ")";
+    }
+
+}

src/test/java/com/fishercoder/_606Test.java

@@ -0,0 +1,41 @@
+package com.fishercoder;
+
+import com.fishercoder.common.classes.TreeNode;
+import com.fishercoder.solutions._606;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Created by stevesun on 6/4/17.
+ */
+public class _606Test {
+    private static _606 test;
+    private static TreeNode t;
+
+    @BeforeClass
+    public static void setup(){
+        test = new _606();
+    }
+
+    @Test
+    public void test1(){
+        t = new TreeNode(1);
+        t.left = new TreeNode(2);
+        t.right = new TreeNode(3);
+        t.left.left = new TreeNode(4);
+        System.out.println("Test1");
+        assertEquals("1(2(4))(3)", test.tree2str(t));
+    }
+
+    @Test
+    public void test2(){
+        t = new TreeNode(1);
+        t.left = new TreeNode(2);
+        t.right = new TreeNode(3);
+        t.left.right = new TreeNode(4);
+        System.out.println("Test2");
+        assertEquals("1(2()(4))(3)", test.tree2str(t));
+    }
+}