免费的b2b平台有哪些_聊城专业网站建设制作_优化公司_网站建设知名公司

题目： 

 

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解析： 

 

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代码： 

private List<String> ret;private StringBuffer path;public List<String> letterCasePermutation(String s) {ret = new ArrayList<>();path = new StringBuffer();dfs(s,0);return ret;}private void dfs(String s, int pos){if(pos == s.length()){ret.add(path.toString());return;} char ch = s.charAt(pos);//改变if(ch < '0' || ch > '9'){char tmp = change(ch);path.append(tmp);dfs(s,pos+1);path.deleteCharAt(path.length()-1);//恢复现场}//不变path.append(ch);dfs(s,pos+1);path.deleteCharAt(path.length()-1);//恢复现场}  private char change(char ch){if(ch >= 'a' && ch <= 'z') return ch -= 32;//小写转大写else return ch += 32;//大写转小写}