LeetCode:Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

Total Accepted: 81548  Total Submissions: 280640  Difficulty: Medium

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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

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java code:

public class Solution {
    public List<String> letterCombinations(String digits) {
        
        String[] mappings = {"0","1","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        
        LinkedList<String> ans = new LinkedList<String>();
        if(digits.length() == 0) return ans;
        ans.add("");
        
        for(int i=0;i<digits.length();i++) {
            int num = digits.charAt(i)-'0';
            while(ans.peek().length() == i) {
                String t = ans.remove();
                for(char c : mappings[num].toCharArray()) {
                    ans.add(t+c);
                }
            }
        }
        return ans;
    }
}