Merge pull request TheAlgorithms#1454 from shellhub/master

Created Problem04 in ProjectEuler

Author: realDuYuanChao

DIRECTORY.md

@@ -203,6 +203,7 @@
 ## ProjectEuler
   * [Problem01](https://github.com/TheAlgorithms/Java/blob/master/ProjectEuler/Problem01.java)
   * [Problem02](https://github.com/TheAlgorithms/Java/blob/master/ProjectEuler/Problem02.java)
+  * [Problem04](https://github.com/TheAlgorithms/Java/blob/master/ProjectEuler/Problem04.java)
 
 ## Searches
   * [BinarySearch](https://github.com/TheAlgorithms/Java/blob/master/Searches/BinarySearch.java)

ProjectEuler/Problem04.java

@@ -0,0 +1,41 @@
+package ProjectEuler;
+
+/**
+ * A palindromic number reads the same both ways.
+ * The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
+ * <p>
+ * Find the largest palindrome made from the product of two 3-digit numbers.
+ * <p>
+ * link: https://projecteuler.net/problem=4
+ */
+public class Problem04 {
+    public static void main(String[] args) {
+
+        assert solution1(10000) == -1;
+        assert solution1(20000) == 19591;   /* 19591 == 143*137 */
+        assert solution1(30000) == 29992;   /* 29992 == 184*163 */
+        assert solution1(40000) == 39893;   /* 39893 == 287*139 */
+        assert solution1(50000) == 49894;   /* 49894 == 494*101 */
+        assert solution1(60000) == 59995;   /* 59995 == 355*169 */
+        assert solution1(70000) == 69996;   /* 69996 == 614*114 */
+        assert solution1(80000) == 79897;   /* 79897 == 733*109 */
+        assert solution1(90000) == 89798;   /* 89798 == 761*118 */
+        assert solution1(100000) == 99999;  /* 100000 == 813*123 */
+    }
+
+    private static int solution1(int n) {
+        for (int i = n - 1; i >= 10000; --i) {
+            String strNumber = String.valueOf(i);
+
+            /* Test if strNumber is palindrome */
+            if (new StringBuilder(strNumber).reverse().toString().equals(strNumber)) {
+                for (int divisor = 999; divisor >= 100; --divisor) {
+                    if (i % divisor == 0 && String.valueOf(i / divisor).length() == 3) {
+                        return i;
+                    }
+                }
+            }
+        }
+        return -1; /* not found */
+    }
+}