Strobogrammatic Number II

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Find all strobogrammatic numbers that are of length = n.

For example,
Given n = 2, return ["11","69","88","96"].

Hint:

1. Try to use recursion and notice that it should recurse with n - 2 instead of n - 1.

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1. https://discuss.leetcode.com/topic/39776/concise-and-simple-c-solution-with-explaination

2. http://www.cnblogs.com/grandyang/p/5200919.html