Given 4 digits `[7, 6, 5, 1]`. Find an expression(s) containing all those digits (once each) and 
arithmetical operations: `+`, `-`, `*`, `/` and parentheses that evaluates to `21`.

Example of possible expression: `(7+6)*(5+1)`

References:
- https://en.wikipedia.org/wiki/Reverse_Polish_notation
- https://leetcode.com/problems/evaluate-reverse-polish-notation/
- https://leetcode.com/problems/expression-add-operators/