add populating next right pointers in each node

Author: tg123

populating-next-right-pointers-in-each-node/README.md

@@ -1,4 +1,33 @@
+## Recursion
 
-## TODO 
-  * write down thinking
+connecting left and right child is easy, just `node.left.next = node.right`
 
+
+Before 
+
+```
+        1
+       /  \
+      2    3
+     / \  / \
+    4  5  6  7
+```
+
+
+After 
+
+```
+        1
+       /  \
+      2 -> 3
+     / \  / \
+    4-> 5 6->7
+```
+
+`5` and `6` shoud be connected。
+
+when connecting `4` and `5`, `2` and `3` is already connected, that is a good news, we can connect `5` and `6` using `2.next`.
+
+```
+node.right.next = node.next.left;
+```