电话号码的字母组合-算法

题目

按电话上数字与字母的对应关系，如2={a,b,c}，3={d,e,f}等，给定一串数字如267，则求出abc,mno,qprs的所有组合，如amq,amp...cor,cos等

思路

遍历都可以用回溯的方式尝试解决，每次遍历结束后，将上一层元素删除，满足长度，则加入到结果中

public List letterCombinations(String digits){
    List combinations = new ArrayList<>();
    if(digits.length()==0){
        return combinations;    
    }
    Map digitLetterMap = new HashMap<>(){{
       put('2',"abc");
       put('3',"def");
       put('4',"ghi");
       put('5',"jkl");
       put('6',"mno");
       put('7',"qprs");
       put('8',"tuv");
       put('9',"wxyz");  
    }};
    backtrack(combinations,digitLetterMap,digits,0,new StringBuffer());
    return combinations;
}

public void backtrack(List combinations,
                        Map digitLetterMap,
                        String digits,
                        int index,StringBuffer combination){
    if(index == digit.legnth()){
        combinations.add(combination.toString());
    }else{
        char digit = digits.charAt(index);
        String letters = digitLetterMap.get(digit);
        int letterCount = letters.legnth();
        for(int i=0;i