Leetcode 17 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.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

循环：输入数字数｛每个之前组成的字符串＋｛每个该次输入数字对应的字母｝｝

def letter_combinations(digits)
    letter = ['','','abc','def','ghi','jkl','mno','pqrs','tuv','wxyz']
    return [] if digits == ''
    ans = [['']]
    digits.chars.each do |x|
        ans << []
        ans[-2].each do |y|
            letter[x.to_i].chars.each {|c| ans[-1] << y+c}
        end
    end
    ans[-1]
end

这里使用了两个变量交替更新，更省空间。

def letter_combinations(digits)
    letter = ['','','abc','def','ghi','jkl','mno','pqrs','tuv','wxyz']
    return [] if digits == ''
    a = ['']
    digits.chars.each do |x|
        b = []
        a.each {|y| letter[x.to_i].chars.each {|c| b << y+c } }
        a = b
    end
    a
end

 

递归：记载当前已生成的字符串

def letter_combinations(digits)
    @letter = ['','','abc','def','ghi','jkl','mno','pqrs','tuv','wxyz']
    return [] if digits == ''
    @ans = []
    comb('',digits,0)
    @ans
end

def comb(str,digits,i)
    @ans << str if str.length == digits.length
    @letter[digits[i].to_i].chars.each {|x| comb(str+x,digits,i+1)}
end