回溯法 | 电话号码的字符组合
BUGGG !!!
/* 字符转数组：ineteger.valueof(string s) 不适用integer.valueof(char ch) 适用 ch-'0'&nbs***bsp;Integer.valueof(s.substring(i,i+1)) */
class Solution {
List<String> s = new ArrayList<>();
public List<String> letterCombinations(String digits) {
if(digits.length()==0) return s;
HashMap<Integer,String> map = new HashMap<>();
map.put(1,"");
map.put(2,"abc");
map.put(3,"def");
map.put(4,"ghi");
map.put(5,"jkl");
map.put(6,"mno");
map.put(7,"pqrs");
map.put(8,"tuv");
map.put(9,"wxyz");
dfs(map,digits,"",0);
return s;
}
public void dfs(HashMap<Integer,String> map, String digits, String ss, int idx){
if(idx==digits.length()){
s.add(ss);
return;
}
// Bug
/*
字符转数组：ineteger.valueof(string s)
不适用integer.valueof(char ch)
适用 ch-'0'&nbs***bsp;Integer.valueof(s.substring(i,i+1))
*/
String st = map.get(digits.charAt(idx)-'0');
for(int i=0;i<st.length();i++){
ss = ss + st.substring(i,i+1);
dfs(map,digits,ss,idx+1);
ss = ss.substring(0,ss.length()-1);
}
}
} 
