add solution of problem 144: binary tree preorder traversal

Author: ppulse

BinaryTreePreorderTraversal144/README.md

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+Given a binary tree, return the *preorder* traversal of its nodes' values.
+
+For example:
+
+Given binary tree `{1,#,2,3}`,
+```
+   1
+    \
+     2
+    /
+   3
+```
+return `[1,2,3]`.
+
+**Note:** Recursive solution is trivial, could you do it iteratively?

BinaryTreePreorderTraversal144/Solution.java

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+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+public class Solution {
+    public List<Integer> preorderTraversal(TreeNode root) {
+        List<Integer> result = new ArrayList<Integer>();
+        
+        if (root != null)
+            preorderTraversal(root, result);
+            
+        return result;
+    }
+    
+    private void preorderTraversal(TreeNode node, List<Integer> result) {
+        result.add(node.val);
+        if (node.left != null)
+            preorderTraversal(node.left, result);
+        
+        if (node.right != null)
+            preorderTraversal(node.right, result);
+    }
+}