江的福从零开始刷算法（递归--电话号码的字母组合）

树形问题，使用递归。

class Solution {
    public String[] str={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    public List<String> letterCombinations(String digits) {
        ArrayList<String> list=new ArrayList<String>();
    
        if(digits.equals("")){
            return list;
        }
        
        String value="";
        findString(digits, 0, value,list);
        return list;

    }
    public void findString(String digits,int index,String value,ArrayList<String> list){
        if (index==digits.length()){
            list.add(value);
            return;
        }
        int num = digits.charAt(index)-'0';
        for(int i =0;i<str[num].length();i++){
            findString(digits, index+1, value+str[num].charAt(i),list);
        }
    }
    
}
           

知识点：

char-> int 字符 -‘0’