Letter Combinations of a Phone Number 电话按键字符的组合@LeetCode

先搞一张电话数字和字符的对应表，再针对给定电话号码DFS回溯

package Level3;import java.util.ArrayList;/** * 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.http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.pngInput:Digit string "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. * */public class S17 {public static void main(String[] args) {System.out.println(letterCombinations("23"));}public static ArrayList<String> letterCombinations(String digits) {String[] ss = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};ArrayList<String> ret = new ArrayList<String>();rec(ret, digits.length(), ss, digits, new StringBuffer());return ret;    }public static void rec(ArrayList<String> ret, int remain, String[] ss, String digits, StringBuffer sb){// 说明对digits遍历结束if(remain == 0){ret.add(sb.toString());return;}String s = ss[digits.charAt(0)-'0'];// 得到digits[0]对应的string s// DFSfor(int i=0; i<s.length(); i++){sb = sb.append(s.charAt(i));rec(ret, remain-1, ss, digits.substring(1), sb);sb.deleteCharAt(sb.length()-1);// 恢复现场}}}

public class Solution {        public List<String> letterCombinations(String digits) {        String[] map = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};        List<String> list = new ArrayList<String>();        rec(digits, map, list, new StringBuilder(), 0);        return list;    }        public void rec(String digits, String[] map, List<String> list, StringBuilder sb, int pos) {        if(pos == digits.length()) {            list.add(sb.toString());            return;        }                String s = map[digits.charAt(pos)-'0'];        for(int i=0; i<s.length(); i++) {            sb.append(s.charAt(i));            rec(digits, map, list, sb, pos+1);            sb.deleteCharAt(sb.length()-1);        }    }}