feat: Solved 17

Author: fghpdf

src/com/fghpdf/LetterCombinationsOfAPhoneNumber/Solution.java

@@ -0,0 +1,48 @@
+package com.fghpdf.LetterCombinationsOfAPhoneNumber;
+
+import java.util.LinkedList;
+import java.util.List;
+
+/**
+ * @author fghpdf
+ * @date 2019/11/14
+ * https://leetcode.com/problems/letter-combinations-of-a-phone-number/
+ *
+ * FIFO
+ * we first build a queue to save letter
+ * when first letter pushed into queue, we should remove it and get it and then
+ * loop next char array
+ * like a -> queue
+ * remove a <- queue
+ * a + d -> queue
+ *
+ * notice the judge is queue.peek().length == i
+ * i is the letter position number
+ * like before first letter join, the i is 0, the queue peek is empty string
+ **/
+public class Solution {
+	public List<String> letterCombinations(String digits) {
+		LinkedList<String> result = new LinkedList<>();
+		if (digits.isEmpty()) {
+			return result;
+		}
+
+		String[] digitsMap = new String[]{"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
+		result.add("");
+		for (int i = 0; i < digits.length(); i++) {
+			int x = Character.getNumericValue(digits.charAt(i));
+			while (result.peek().length() == i) {
+				String temp = result.remove();
+				for (char s : digitsMap[x].toCharArray()) {
+					result.add(temp + s);
+				}
+			}
+		}
+		return result;
+	}
+
+	public static void main(String[] args) {
+		Solution sol = new Solution();
+		System.out.println(sol.letterCombinations("235"));
+	}
+}