Array/String:
- 1768. Merge Strings Alternately: use StringBuilder
- 1071. Greatest Common Divisor of Strings:
if str1 and str2 can be divided, then [math]\displaystyle{ str1 + str2 = str2 + str1 \gcd(a,b)=\gcd(b,a \bmod b) }[/math] just get the substring of gcd length