Add Solution.java to problems 0637

Author: huaxu1024

solution/0637. Average of Levels in Binary Tree/Solution.java

@@ -0,0 +1,35 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+
+    public List<Double> averageOfLevels(TreeNode root) {
+        if (root == null) return null;
+
+        List<Double> res = new ArrayList<>();
+        LinkedList<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+
+        while (!queue.isEmpty()) {
+            double sum = 0, size = queue.size();
+            for (int i = 0; i < size; i ++) {
+                TreeNode node = queue.poll();
+                sum += node.val;
+                if (node.left != null) {
+                    queue.offer(node.left);
+                }
+                if (node.right != null) {
+                    queue.offer(node.right);
+                }
+            }
+            res.add(sum / size);
+        }
+        return res;
+    }
+}