[March-week2] Flip card

1. how to get gcd, lcm

   • using Euclidean algorithm
     solution

     ``` C++ class Solution { public : int gcd(int a, int b) { int t = 0; while(b != 0) { t = b; b = a%b; a = t; } return a } int lcm(int a, int b) { return (a * b) / gcd(a, b); } } ```
2. Elvis Operation
   • using Elvis Operation return str?.length if null then -1
     Solution

     ```kotlin val lenOfString = str?.length ?: -1 ```
3. Safe Call
   • Get string length using safe Call variable name is str
     Solution

     ```kotlin str?.length ```