Add Solution2.java to problems 0017

Author: huaxu1024

solution/0017.Letter Combinations of a Phone Number/Solution2.java

@@ -0,0 +1,41 @@
+class Solution {
+
+    public List<String> letterCombinations(String digits) {
+        Map<String, String> map = new HashMap<>();
+        for (int i = 2, index = 0; i < 7; i ++) {
+            for (int j = 0; j < 3; j ++) {
+                map.put(String.valueOf(i) + String.valueOf(j), (char)('a' + (index++)) + "");
+            }
+        }
+        map.put("70", "p");
+        map.put("71", "q");
+        map.put("72", "r");
+        map.put("73", "s");
+
+        map.put("80", "t");
+        map.put("81", "u");
+        map.put("82", "v");
+
+        map.put("90", "w");
+        map.put("91", "x");
+        map.put("92", "y");
+        map.put("93", "z");
+        List<String> res = new ArrayList<>();
+        if (digits.length() > 0) {
+            solution(res, map, digits, "", 0);
+        }
+        return res;
+    }
+
+    private void solution(List<String> res, Map<String, String> map, String digits, String target, int index) {
+        if (index == digits.length()) {
+            res.add(target);
+            return;
+        }
+        String snet = String.valueOf(digits.charAt(index));
+        int length = ("7".equals(snet) || "9".equals(snet)) ? 4 : 3;
+        for (int i = 0; i < length; i++) {
+            solution(res, map, digits, target + map.get(snet + i), index + 1);
+        }
+    }
+}