Adding 451. Sort Characters By Frequency to problems page

[YonashPetit]

I'm adding the "451. Sort Characters By Frequency" Leetcode problem to the problems page (in English).

[azl397985856]

1. filename should be problems/451.sort-characters-by-frequency.en.md
2. change 'Summary of Key Operations:' to 'Key Points'
3. remove ''' in the code section
4. 'Map Hash Map from left to right, during traverse, group nodes in k, then reverse each group. How to reverse a linked list given start, end node?' I don't quite understand what did you say, could you please delete it or use a more clear way
5. detail of 'bulding the Frequency Dictionary' is unnessary. delete it to keep it brief. (maybe you can try Counter,it's much more brief)
6. complexity is wrong. it's O(biggest + n) for both time and space
7. final step is 'adding the link in README.md and SUMMARY.md'