17. Letter Combinations of a Phone Number

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Given a digit string, 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.

Input:Digit string "23"Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

List的remove() 方法从头开始向外删除对象，add添加到后边。最巧妙的是利用了ans.peek().length()==i 来判断弹出的次数

先建立一个字符串数组，可以取每个数字对应的字符组成的字符串，将字符串变成字符数组进行遍历向原来的数组添加

public class Solution {  public List<String> letterCombinations(String digits) {    LinkedList<String> ans = new LinkedList<String>();    String[] mapping = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};    ans.add("");    for(int i =0; i<digits.length();i++){        int x = Character.getNumericValue(digits.charAt(i));        while(ans.peek().length()==i){            String t = ans.remove();            for(char s : mapping[x].toCharArray())                ans.add(t+s);        }    }    return ans;}}

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