java: add «129. Sum Root to Leaf Numbers»

Signed-off-by: Matej Focko <mfocko@redhat.com>

java/sum-root-to-leaf-numbers.java

@@ -0,0 +1,22 @@
+class Solution {
+  private boolean isLeaf(TreeNode node) {
+    return node != null && node.left == null && node.right == null;
+  }
+
+  private int sumNumbers(TreeNode node, int number) {
+    if (node == null) {
+      return 0;
+    }
+
+    number = 10 * number + node.val;
+    if (isLeaf(node)) {
+      return number;
+    }
+
+    return sumNumbers(node.left, number) + sumNumbers(node.right, number);
+  }
+
+  public int sumNumbers(TreeNode root) {
+    return sumNumbers(root, 0);
+  }
+}