24.Letter Combinations of a Phone Number(电话号对应的字符组合)
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Level:
??Medium
题目描述:
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"].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.
思路分析:
??建立hashmap将每个数字对应的字符映射存放在map中,利用递归回溯解决问题。
代码:
public class Solution{
public List<String>letterCombinations(String digits){
List<String>res=new ArrayList<>();
if(digits==null||digits.equals(""))
return res;
HashMap<Character,char[]>map=new HashMap<>();
map.put('0',new char[]{});
map.put('1',new char[]{});
map.put('2',new char[]{'a','b','c'});
map.put('3',new char[]{'d','e','f'});
map.put('4',new char[]{'g','h','i'});
map.put('5',new char[]{'j','k','l'});
map.put('6',new char[]{'m','n','o'});
map.put('7',new char[]{'p','q','r','s'});
map.put('8',new char[]{'t','u','v'});
map.put('9',new char[]{'w','x','y','z'});
StringBuilder str=new StringBuilder();
findComb(digits,map,res,str);
return res;
}
public void findComb(String digits,HashMap<Character,char[]>map,List<String>res,StringBuilder str){
if(str.length==digits.length){//str的长度等于digits的长度证明是其中一个结果
res.add(str.toString());
return;
}
for(char c:map.get(digits.charAt(str.length()))){
str.append(c);
findComb(digits,map,res,str);//递归回溯找出所有的结果
str.deleteCharAt(str.length()-1);//每找出一个结果后,str的长度减一,添加下一种可能
}
}
}
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