穷举vs暴搜vs深搜vs回溯vs剪枝系列一＞电话号码的字母组合

题目：

 

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解析： 

 

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代码： 

private String[] hash = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};private StringBuffer path;private List<String> ret;public List<String> letterCombinations(String digits) {ret = new ArrayList<>();path = new StringBuffer();if(digits.length() == 0) return ret;dfs(digits,0);return ret;} private void dfs(String digits,int pos){if(digits.length() == pos){ret.add(path.toString());return;} char[] cur = hash[digits.charAt(pos)-'0'].toCharArray();for(int i = 0; i < cur.length; i++){path.append(cur[i]);dfs(digits, pos+1);//恢复现场 //deleteCharAt (int指数) 移除此序列中指定位置的字符path.deleteCharAt(path.length()-1);}}