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algorithms/permutation.md

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+Permutation
+==
+
+- You are given a 7 digit phone number, and you should find all possible letter combinations based on the digit-to-letter mapping on numeric pad and return only the ones that have valid match against a given dictionary of words.
+- Give all possible letter combinations from a phone number.
+- Generate all subsets of a string.
+- Print all possible `N` pairs of balanced parentheses.
+  - E.g. when `N` is `2`, the function should print `(())` and `()()`.
+  - E.g. when `N` is `3`, we should get `((()))`, `(()())`, `(())()`, `()(())`, `()()()`.
+  - [Source](http://blog.gainlo.co/index.php/2016/12/23/uber-interview-questions-permutations-parentheses/)
+- Given a list of arrays, return a list of arrays, where each array is a combination of one element in each given array.
+   - E.g. If the input is `[[1, 2, 3], [4], [5, 6]]`, the output should be `[[1, 4, 5], [1, 4, 6], [2, 4, 5], [2, 4, 6], [3, 4, 5], [3, 4, 6]]`.