Update 0242-valid-anagram.cpp

[Steven-Yiran]

The original iteration of the unordered_map created unnecessary creation of additional keys.

• File(s) Modified: 0242-valid-anagram.cpp
• Language(s) Used: cpp
• Submission URL: https://leetcode.com/problems/valid-anagram/submissions/968297894/

Important

Please make sure the file name is lowercase and a duplicate file does not already exist before merging.