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问题：
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"].
原来的问题是给定一个由数字组成的字符串，求这些数字在电话按键上对应的字母的所有组合。我们考虑使用动态规划来解决这个问题。假设这个字符串中的数字从左到右依次为C1, C2, ..., Cn，共n个数字，现在考虑能否把这个问题转化成一个规模更小，但是性质类似的问题，即当已经找到给定字符串包含C1, C2, ..., Cn-1这前n-1个数字的字母组合时，怎样求出再加一个数字的字母组合
1 public class Solution1 { 2 public List<String> letterCombinations(String digits) { 3 String[] mapping = new String[]{"", "", "abc", "def", "ghi", 4 "jkl", "mno", "pqrs", "tuv", "wxyz"}; 5 List<String> lastList = new ArrayList<String>(); 6 if(digits == null || digits.length() == 0) { 7 return lastList; 8 } 9 lastList.add(""); 10 List<String> workList = new ArrayList<String>(); 11 for(int i = 0; i < digits.length(); i++) { 12 int digit = Integer.parseInt(String.valueOf(digits.charAt(i))); 13 for(String str : lastList) { 14 for(int j = 0; j < mapping[digit].length(); j++) { 15 StringBuilder sb = new StringBuilder(str); 16 sb.append(mapping[digit].charAt(j)); 17 workList.add(sb.toString()); 18 } 19 } 20 21 List<String> tmp = lastList; 22 lastList = workList; 23 workList = tmp; 24 25 workList.clear(); 26 } 27 28 return lastList; 29 } 30 }
[Leetcode] Letter Combinations of a Phone Number
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原文地址：http://www.cnblogs.com/seagoo/p/5000528.html
