Add Java solutions for leetcode 405.

Author: yanglr

java-leetcode_vscode/Leetcode405a.java

@@ -0,0 +1,47 @@
+import java.util.*;
+
+/** 如果有多种方法, 可以将文件名和class 名字都改为 Leetcode1_method2或Leetcode1b 这种格式 */
+class Leetcode405a {
+        public String toHex(int num) {
+            String res = "";
+            long N = num; // 预处理
+            if (N == 0)
+                return "0";
+            HashMap<Integer, Character> dict = new HashMap<Integer, Character>() {
+                {
+                    put(0, '0');
+                    put(1, '1');
+                    put(2, '2');
+                    put(3, '3');
+                    put(4, '4');
+                    put(5, '5');
+                    put(6, '6');
+                    put(7, '7');
+                    put(8, '8');
+                    put(9, '9');
+                    put(10, 'a');
+                    put(11, 'b');
+                    put(12, 'c');
+                    put(13, 'd');
+                    put(14, 'e');
+                    put(15, 'f');
+                }
+            };
+            if (N < 0) N = 4294967296L + N; /* 4294967296L就是 0x0000000100000000(16^8=2^32), Java中
+                                             * 不使用BigInteger无法存储该数, 只能hard code在这里了
+                                             */
+            while (N > 0) {
+                long lastDigit = N % 16;
+                N /= 16;
+                res = dict.get((int) lastDigit) + res;
+            }
+            return res;
+        }
+        
+    public static void main(String[] args) {
+        Leetcode405a sol = new Leetcode405a();
+        int num = 7;
+        String res = sol.toHex(num);        
+        System.out.println(res);
+    }
+}

java-leetcode_vscode/Leetcode405b.java

@@ -0,0 +1,24 @@
+/** 如果有多种方法, 可以将文件名和class 名字都改为 Leetcode1_method2或Leetcode1b 这种格式 */
+class Leetcode405b {
+    public String toHex(int num) {
+        String res = "";
+        long N = num;     // 预处理
+        if (N == 0) return "0";
+        String dict = "0123456789abcdef";
+        if (N < 0) N = 4294967296L + N;  /* 4294967296L就是 0x0000000100000000(16^8=2^32), Java中不使用BigInteger无法存储, 只能hard code在这里了 */
+        while (N > 0)
+        {
+            long lastDigit = N % 16;
+            N /= 16;
+            res = dict.charAt((int)lastDigit) + res;
+        }
+        return res;             
+    }
+        
+    public static void main(String[] args) {
+        Leetcode405a sol = new Leetcode405a();
+        int num = 7;
+        String res = sol.toHex(num);        
+        System.out.println(res);
+    }
+}