Micro-Drill #193 — Letter combinations of phone number

def letterCombinations(digits):
    if not digits: return []
    phone = {'2':'abc','3':'def','4':'ghi','5':'jkl',
             '6':'mno','7':'pqrs','8':'tuv','9':'wxyz'}
    res = []
    def backtrack(i, path):
        if i == len(digits):
            res.append(''.join(path))
            return
        for ch in phone[digits[i]]:
            path.append(ch)
            backtrack(i + 1, path)
            path.pop()
    backtrack(0, [])
    return res