add 1556

Author: fishercoder1534

README.md

@@ -8,6 +8,7 @@ _If you like this project, please leave me a star._ ★
 
 |  #  |      Title     |   Solutions   | Video  | Difficulty  | Tag                   
 |-----|----------------|---------------|--------|-------------|-------------
+|1556|[Thousand Separator](https://leetcode.com/problems/thousand-separator/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1556.java) ||Easy|String|
 |1551|[Minimum Operations to Make Array Equal](https://leetcode.com/problems/minimum-operations-to-make-array-equal/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1551.java) | [:tv:](https://youtu.be/A-i2sxmBqAA)|Medium|Math|
 |1550|[Three Consecutive Odds](https://leetcode.com/problems/three-consecutive-odds/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1550.java) | |Easy|Array|
 |1545|[Find Kth Bit in Nth Binary String](https://leetcode.com/problems/find-kth-bit-in-nth-binary-string/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_1545.java) | |Medium|String|

src/main/java/com/fishercoder/solutions/_1556.java

@@ -0,0 +1,22 @@
+package com.fishercoder.solutions;
+
+public class _1556 {
+    public static class Solution1 {
+        public String thousandSeparator(int n) {
+            String str = Integer.toString(n);
+            StringBuilder sb = new StringBuilder();
+            for (int i = str.length() - 1, j = 1; i >= 0; i--, j++) {
+                sb.append(str.charAt(i));
+                j++;
+                if (j % 3 == 0) {
+                    sb.append(".");
+                }
+            }
+            String result = sb.reverse().toString();
+            if (result.charAt(0) == '.') {
+                result = result.substring(1);
+            }
+            return result;
+        }
+    }
+}

src/test/java/com/fishercoder/_1556Test.java

@@ -0,0 +1,22 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._1556;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static junit.framework.TestCase.assertEquals;
+
+public class _1556Test {
+    private static _1556.Solution1 solution1;
+
+    @BeforeClass
+    public static void setup() {
+        solution1 = new _1556.Solution1();
+    }
+
+    @Test
+    public void test1() {
+        assertEquals("987", solution1.thousandSeparator(987));
+    }
+
+}