題目連結

解題思路：
回溯

提交程式碼：

class Solution {
    public List<String> letterCombinations(String digits) {
    	List<String> res=new ArrayList<String>();
    	if(digits.length()==0)  return res;
    	String basestr= "";

    	findCombination(res,digits,0,basestr);
        
        return
 
 res;
    }
    
    public void findCombination(List<String> res,
    		String digits,int p,String basestr) {
    	//res:result   digits:the input digits string  
    	//p:the position of char in digits
    	String[] chars= {
    			"0","1","abc","def","ghi","jkl",
    			"mno","pqrs","tuv","wxyz"
 

    	};

    	if(p==digits.length()) {
    		res.add(basestr);
    		return;
    	}
    	int index=digits.charAt(p)-'0';
    	for(int i=0;i<chars[index].length();i++) {
    		basestr+=chars[index].charAt(i);
    		findCombination(res,digits,p+1,basestr);
    		basestr=basestr.substring(0, p);
    	}
 

    }
}

執行結果：