day76（2.4）——leetcode面试经典150

17. 电话号码的字母组合

17. 电话号码的字母组合

题目：

题解：

class Solution { List<String> ls; Map<Character, List<Character>> map; public List<String> letterCombinations(String digits) { map = new HashMap<>(); Character ch = 'a'; for(Character c='2';c<='9';c++) { List<Character> list = new ArrayList<>(); list.add(ch++); list.add(ch++); list.add(ch++); if(c == '7' || c == '9') { list.add(ch++); } map.put(c, list); } ls = new ArrayList<>(); dfs(0,digits,new StringBuilder()); return ls; } void dfs(int x, String digits, StringBuilder sb) { if(x>=digits.length()) { ls.add(sb.toString()); return; } List<Character> cs = map.get(digits.charAt(x)); for(int j=0;j<cs.size();j++) { dfs(x+1, digits, sb.append(cs.get(j))); sb.deleteCharAt(sb.length() - 1); } } }