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## Problem

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.

Example 1:

Input: digits = "23"


Example 2:

Input: digits = ""
Output: []


Example 3:

Input: digits = "2"
Output: ["a","b","c"]


Constraints:

• 0 <= digits.length <= 4
• digits[i] is a digit in the range [‘2’, ‘9’].

## Code

• valueOf 和 getNumericValue
• valueOf 会返回数字对应的 ascii 码
public class test {
public static void main(String [] args){
char c = '5';

System.out.println(Character.getNumericValue(c));
System.out.println(Integer.valueOf(c));

// String 不会受到影响
String s = "5";
System.out.println(Integer.valueOf(s));
}
}
// 输出
// 5
// 53
// 5

class Solution {
public List<String> letterCombinations(String digits) {
if(digits == null || digits.length() == 0) return new ArrayList<>();

String[] dict = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

queue.offer("");

for(int i = 0; i < digits.length(); i++) {
int index = digits.charAt(i) - '0';
String curr = dict[index];
while(queue.peek().length() == i) {
String temp = queue.poll();
for(char c : curr.toCharArray()){
queue.offer(temp + String.valueOf(c));
}
}
}

return new ArrayList<>(queue);
}
}