Letter Combinations of a Phone Number
最新更新请见:https://yanjia.me/zh/2019/01/...
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"]
. Note: Although the above answer is in lexicographical order, your answer could be in any order you want.
深度优先搜索
复杂度
时间 O(N) 空间 O(N) 递归栈空间
思路
首先建一个表,来映射号码和字母的关系。然后对号码进行深度优先搜索,对于每一位,从表中找出数字对应的字母,这些字母就是本轮搜索的几种可能。
注意
- 用StringBuilder构建临时字符串
- 当临时字符串为空时,不用将其加入结果列表中
代码
public class Solution {
List<String> res;
public List<String> letterCombinations(String digits) {
// 建立映射表
String[] table = {" ", " ", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
StringBuilder tmp = new StringBuilder();
res = new LinkedList<String>();
helper(table, 0, tmp, digits);
return res;
}
private void helper(String[] table, int idx, StringBuilder tmp, String digits){
if(idx == digits.length()){
// 找到一种结果,加入列表中
if(tmp.length()!=0) res.add(tmp.toString());
} else {
// 找出当前位数字对应可能的字母
String candidates = table[digits.charAt(idx) - '0'];
// 对每个可能字母进行搜索
for(int i = 0; i < candidates.length(); i++){
tmp.append(candidates.charAt(i));
helper(table, idx+1, tmp, digits);
tmp.deleteCharAt(tmp.length()-1);
}
}
}
}
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