Morris - inorder traversal running O(N), without extra storage

http://www.quora.com/Why-does-the-Morris-in-order-traversal-algorithm-have-O-n-time-complexity
is the detailed.

https://codeoverflow.wordpress.com/tag/morris-inorder-traversal/ has explanations + full c++ code

http://www.geeksforgeeks.org/inorder-tree-traversal-without-recursion-and-without-stack/ has C code

http://n00tc0d3r.blogspot.sg/2013/09/inorder-binary-tree-traversal-with.html