add solution of problem 145: binary tree post order traversal

Author: ppulse

BinaryTreePostorderTraversal145/README.md

@@ -0,0 +1,14 @@
+Given a binary tree, return the *postorder* traversal of its nodes' values.
+
+For example:
+Given binary tree `{1,#,2,3}`,
+```
+   1
+    \
+     2
+    /
+   3
+```
+return `[3,2,1]`.
+
+Note: Recursive solution is trivial, could you do it iteratively?

BinaryTreePostorderTraversal145/Solution.java

@@ -0,0 +1,29 @@
+/**
+ * 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> postorderTraversal(TreeNode root) {
+        List<Integer> result = new ArrayList<Integer>();
+        
+        if (root != null)
+            postorderTraversal(root, result);
+        
+        return result;
+    }
+    
+    private void postorderTraversal(TreeNode node, List<Integer> result) {
+        if (node.left != null)
+            postorderTraversal(node.left, result);
+        
+        if (node.right != null)
+            postorderTraversal(node.right, result);
+        
+        result.add(node.val);
+    }    
+}