17. Letter Combinations of a Phone Number
Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.
Example:
Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.
難度:medium
題目:
給定字串包含數字2到9,返回所有字母組合。數字字母的對應關係如下圖所示。注意1不與任何字母對應。
思路:
組合,遞迴
Runtime: 2 ms, faster than 80.94% of Java online submissions for Letter Combinations of a Phone Number.
Memory Usage: 26.4 MB, less than 14.87% of Java online submissions for Letter Combinations of a Phone Number.
class Solution { public List<String> letterCombinations(String digits) { List<String> result = new ArrayList(); if (digits.isEmpty()) { return result; } Map<Character, String> mcs = new HashMap<>(); mcs.put('2', "abc"); mcs.put('3', "def"); mcs.put('4', "ghi"); mcs.put('5', "jkl"); mcs.put('6', "mno"); mcs.put('7', "pqrs"); mcs.put('8', "tuv"); mcs.put('9', "wxyz"); StringBuilder s = new StringBuilder(); letterCombinations(digits, 0, mcs, result, s); return result; } private void letterCombinations(String digits, int digitIdx, Map<Character, String> mcs, List<String> result, StringBuilder s) { if (s.length() == digits.length()) { result.add(s.toString()); return; } char[] cs = digits.toCharArray(); for (int i = digitIdx; i < cs.length; i++) { String str = mcs.get(cs[i]); for (int j = 0; j < str.length(); j++) { s.append(str.charAt(j)); letterCombinations(digits, i + 1, mcs, result, s); s.deleteCharAt(s.length() - 1); } } } }