Permutation Summary

Method:
Iteration and recursion.

Like the code in this post, they are from me and easier for me to understand and memorize.
http://mashijie.blogspot.ca/2015/07/string-permutation.html

Below are newer posts and borrowed from the Net. I just could not believe that I would have totally forgotten what I come up a year earlier.

In iteration, it loops through the queue to continuously build it up, An example can be found here.
http://mashijie.blogspot.ca/search?q=subsets

In recursion, it tries to work the resolved with unused elements. An example can be found here:
http://mashijie.blogspot.ca/2016/07/permutations.html