Letter Combinations of a Phone Number


Given a digit string, return all possible letter combinations that the number could represent.

 A mapping of digit to letters (just like on the telephone buttons) is given below.


Solution: 

 Not hard. Use recursion or iterative. The following code passes LeetCode Online Large Judge.

2013-05-19: iterative solution. Inefficient due to additional vector operations. Need to be optimized.

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