"that are all of the same length" is the key. This statement makes everything much simpler. And, please take care that L may contain duplicated strings.
My main reference is: http://yucoding.blogspot.com/2013/09/leetcode-question-106-substring-with.html
Remember: feel the problem first.. then look up your armory
LeetCode "Substring with Concatenation of All Words",布布扣,bubuko.com
LeetCode "Substring with Concatenation of All Words"
原文：http://www.cnblogs.com/tonix/p/3899211.html