电话号码的字母组合——LeetCode

这个挺容易想明白的，就是写代码的时候，用回溯法，涉及到递归调用的时候有点绕

 代码:

public class LetterCombinations {
    public static void main(String[] args) {
        System.out.println(letterCombinations("234"));
    }

    public static List letterCombinations(String digits) {
        List list = new ArrayList<>();
        Map hashMap = new HashMap<>();
        hashMap.put(2, "abc");
        hashMap.put(3, "def");
        hashMap.put(4, "ghi");
        hashMap.put(5, "jkl");
        hashMap.put(6, "mno");
        hashMap.put(7, "pqrs");
        hashMap.put(8, "tuv");
        hashMap.put(9, "wxyz");

        if (digits.length() > 0) {
            backTrace(hashMap, 0, digits, list, new StringBuffer());
        }

        return list;
    }

    private static void backTrace(Map hashMap, int index, String digits, List list, StringBuffer stringBuffer) {
        if (digits.length() == index) {
            list.add(stringBuffer.toString());
        } else {
            String temp = hashMap.get(digits.charAt(index) - 48); //获取数字对应的字符串
            for (int i = 0; i < temp.length(); i++) {
                stringBuffer.append(temp.charAt(i));
                backTrace(hashMap,index+1,digits,list,stringBuffer);
                stringBuffer.deleteCharAt(index); //这句不容易想到
            }
        }
    }
}