java: add «145. Binary Tree Postorder Traversal»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/TreeNode.java

@@ -0,0 +1,17 @@
+public class TreeNode {
+  int val;
+  TreeNode left;
+  TreeNode right;
+
+  TreeNode() {}
+
+  TreeNode(int val) {
+    this.val = val;
+  }
+
+  TreeNode(int val, TreeNode left, TreeNode right) {
+    this.val = val;
+    this.left = left;
+    this.right = right;
+  }
+}

java/binary-tree-postorder-traversal.java

@@ -0,0 +1,20 @@
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+  private ArrayList<Integer> postorderTraversal(TreeNode node, ArrayList<Integer> l) {
+    if (node == null) {
+      return l;
+    }
+
+    postorderTraversal(node.left, l);
+    postorderTraversal(node.right, l);
+
+    l.add(node.val);
+    return l;
+  }
+
+  public List<Integer> postorderTraversal(TreeNode root) {
+    return postorderTraversal(root, new ArrayList<Integer>());
+  }
+}