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"].
这里很容易想到要递归调用，直接给出代码如下：
public List<String> letterCombinations(String digits) { return getList(digits); } public static List<String> getList(String digits) { if (0 == digits.length()) { return new ArrayList<String>(0); } else if (1 == digits.length()) { return DICTIONARY.get(digits); } else { List<String> result = new ArrayList<String>(); List<String> fristStr = DICTIONARY.get("" + digits.charAt(0)); List<String> preResult = getList(digits.substring(1, digits.length())); for (String letter : fristStr) { for (String lett : preResult) { result.add(letter + lett); } } return result; } } private static final Map<String, List<String>> DICTIONARY = new HashMap<String, List<String>>() {{ put("0", Arrays.asList("")); put("1", Arrays.asList("")); put("2", Arrays.asList("a", "b", "c")); put("3", Arrays.asList("d", "e", "f")); put("4", Arrays.asList("g", "h", "i")); put("5", Arrays.asList("j", "k", "l")); put("6", Arrays.asList("m", "n", "o")); put("7", Arrays.asList("p", "q", "r", "s")); put("8", Arrays.asList("t", "u", "v")); put("9", Arrays.asList("w", "x", "y", "z")); }};
最早的时候觉得写字典表麻烦，还萌萌哒的写了个函数生成字典（花费的时间更多），其实面对这一类固定的数据，不管从编码时间还是程序运行效率上来讲，直接写静态变量都是更好的选择。
private static HashMap<String, List<String>> getLetterMap() { HashMap<String, List<String>> letterMap = new HashMap<String, List<String>>(); letterMap.put("0", new ArrayList<String>(0)); letterMap.put("1", new ArrayList<String>(0)); int letterIndex = 97; int index = 0; for (int i = 2; i < 10; i++) { List<String> letters = new ArrayList<String>(); if (String.valueOf(i).equals("7") || String.valueOf(i).equals("9")) { index = 4; } else { index = 3; } for (int j = 0; j < index; j++) { letters.add(String.valueOf((char) letterIndex)); letterIndex++; } letterMap.put(String.valueOf(i), letters); } return letterMap; }
17. Letter Combinations of a Phone Number
原文：https://www.cnblogs.com/lyInfo/p/9097835.html