标签：str 维护 like ssi medium div 数组下标 put hat
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
use hashmap to store digit and characters accordingly.
at ith position of input:
use queue to store the combinations of the first i digits, when adding new digits, just need to get every combination in the queue and add the new digit on each one.
class Solution { public List<String> letterCombinations(String digits) { List<String> list=new ArrayList<>(); if(digits.length()==0) return list; Map<Character, String> map=new HashMap<>(); 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"); Queue<String> queue=new LinkedList<>(); queue.offer(""); for(char c:digits.toCharArray()){ int num=queue.size(); for(int i=0;i<num;i++){ String str=queue.poll(); for(char x:map.get(c).toCharArray()){ queue.offer(str+x); } } } while(!queue.isEmpty()){ list.add(queue.poll()); } return list; } }
不要老是想着hashmap！
如果index可以用数组下标表示就避免用hashmap，数组的维护比hashmap要容易快捷的多
class Solution { public List<String> letterCombinations(String digits) { List<String> list=new ArrayList<>(); if(digits.length()==0) return list; String[] record=new String[]{null, null, "abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"}; Queue<String> queue=new LinkedList<>(); queue.offer(""); for(char c:digits.toCharArray()){ int num=queue.size(); for(int i=0;i<num;i++){ String str=queue.poll(); for(char x:record[c-‘0‘].toCharArray()){ queue.offer(str+x); } } } while(!queue.isEmpty()){ list.add(queue.poll()); } return list; } }
backtracking:
class Solution { public List<String> letterCombinations(String digits) { List<String> list=new ArrayList<>(); if(digits.length()==0) return list; String[] record=new String[]{null, null, "abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"}; getCombination(digits, record, "", 0, list); return list; } public void getCombination(String digits, String[] record, String str, int index, List<String> list){ if(index>=digits.length()){ list.add(str); return; } for(int i=0;i<record[digits.charAt(index)-‘0‘].length();i++){ getCombination(digits, record, str+record[digits.charAt(index)-‘0‘].charAt(i), index+1, list); } } }
leetcode 17-Letter Combinations of a Phone Number(medium)
标签：str 维护 like ssi medium div 数组下标 put hat
原文地址：https://www.cnblogs.com/yshi12/p/9690104.html
