【算法三十三】17. 电话号码的字母组合

17. 电话号码的字母组合

回溯：

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<>();
        if(digits.length() == 0){
            return ans;
        }
        Map<Character,String> hashmap = Map.of(
            '2',"abc",
            '3',"def",
            '4',"ghi",
            '5',"jkl",
            '6',"mno",
            '7',"pqrs",
            '8',"tuv",
            '9',"wxyz"
        );
        StringBuffer sb = new StringBuffer();
        backtrack(ans,hashmap,digits,0,sb);
        return ans;
    }

    public void backtrack(List<String> ans,Map<Character,String> hashmap,String digits,int index,StringBuffer sb){
        if(index == digits.length()){
            ans.add(sb.toString());
        }
        else{
            char digit = digits.charAt(index);
            String letters = hashmap.get(digit);
            for(int i = 0;i<letters.length();i++){
                sb.append(letters.charAt(i));
                backtrack(ans,hashmap,digits,index+1,sb);
                sb.deleteCharAt(index);
            }
        }
    }
}

时间复杂度：O(3^m×4^n)

空间复杂度：O(m+n)