Fixed error in example matrix

Update three-in-one.mdx (#20)

edited word freqs

numberswapper edits

Solutions for 16.1 and 16.2 (#19)

elaborated on explanation

fixed url

added more explanation

solution for bst-sequence 4.9 (#17)

adds solution for 4.7 and 4.8 (#14)

revised check balanced

add comment clarifying capitalization check

Update is-unique.mdx (#16)

rewrote palindrome permutation explanation

rewrote explanation for urlify