add populating next right pointers ii

Author: tg123

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

@@ -1,4 +1,30 @@
+## Improved version of [Populating Next Right Pointers in Each Node](../populating-next-right-pointers-in-each-node)
 
-## TODO 
-  * write down thinking
+This time, finding next is a bit difficult.
+`node.right.next = node.next.left` is no longer working.
 
+But parent's next is also useful, and only need to seach a little more nodes.
+
+```
+    1
+   /  \
+  2    3
+   \    \
+    4    5
+   /    / \
+  6    7   8
+```
+
+ * `6.next` = `7` = `5.left`  = `4.next.left`
+ * `4.next` = `5` = `3.right` = `2.next.right`
+
+Searching path is like
+
+ * node's parent's left
+ * node's parent's right
+ * take node's parent's next as parent and search again
+
+
+Note:
+
+`connect(right)` before `connect(left)`, because `left nodes` depend on `right nodes' next`