Update and rename collatz.md to collatz-algorithm.md

snippets/collatz-algorithm.md

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+### Collatz algorithm
+
+If `n` is even, return `n/2`. Otherwise  return `3n+1`.
+
+```js
+const collatz = n => (n % 2 == 0) ? (n/2) : (3*n+1); 
+// collatz(8) --> 4
+// collatz(5) --> 16
+```