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”].

一刷ac

解题思路：利用递归的方法可以，采用前一段字符串来生成当前字符串的映射；非递归可以用队列存结果，每次利用队头生成新的结果。

public class Solution {
public List<String> letterCombinations(String digits) {
List<String> res = new ArrayList<String>();
if(digits == null || digits.length() == 0) return res;
char[] chars = digits.toCharArray();
HashMap<Character, String> map = new HashMap<Character, String>();
map.put('2', "abc");
map.put('3', "def");
map.put('4', "ghi");
map.put('5', "jkl");
map.put('6', "mno");
map.put('7', "pqrs");
map.put('8', "tuv");
map.put('9', "wxyz");
if(chars.length == 1){
String tmp = map.get(chars[0]);
for(int i = 0; i < tmp.length(); i++){
res.add(String.valueOf(tmp.charAt(i)));
}
return res;
}
String oldstr = digits.substring(0, digits.length()-1);
List<String> oldres = letterCombinations(oldstr);
String laststr = map.get(chars[chars.length-1]);
for(String s : oldres){
for(int i = 0; i < laststr.length(); i++){
String tmp = s;
tmp = tmp + laststr.charAt(i);
res.add(tmp);
}
}
return res;
}
}

public class Solution {
public List<String> letterCombinations(String digits) {
List<String> res = new ArrayList<String>();
if(digits == null || digits.length() == 0) return res;
LinkedList<String> queue = new LinkedList<String>();
queue.offer("");
HashMap<Character, String> map = new HashMap<Character, String>();
map.put('2', "abc");
map.put('3', "def");
map.put('4', "ghi");
map.put('5', "jkl");
map.put('6', "mno");
map.put('7', "pqrs");
map.put('8', "tuv");
map.put('9', "wxyz");
for(int i = 0; i < digits.length(); i++){
String str = map.get(digits.charAt(i));
int size = queue.size();
for(int j = 0; j < size; j++){
String s = queue.pollFirst();
for(int k = 0; k < str.length(); k++){
String tmp = s + str.charAt(k);
queue.offer(tmp);
}
}
}
res.addAll(queue);
return res;
}
}

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