solves #2815: Max Pair Sum in an Array in java

Author: anishLearnsToCode

Diff for: README.txt

@@ -843,4 +843,7 @@
 | 2788 | [Split Strings by Separator](https://leetcode.com/problems/split-strings-by-separator)                                                                                 |                                                                                   [![Java](assets/java.png)](src/SplitStringsBySeparator.java)                                                                                                                                               |                                                                                                                                   |
 | 2798 | [Number of Employees Who Met the Target](https://leetcode.com/problems/number-of-employees-who-met-the-target)                                                         |                                                                                   [![Java](assets/java.png)](src/NumberOfEmployeesWhoMetTheTarget.java)                                                                                                                                               |                                                                                                                                   |
 | 2806 | [Account Balance After Rounded Purchase](https://leetcode.com/problems/account-balance-after-rounded-purchase)                                                         |                                                                                   [![Java](assets/java.png)](src/AccountBalanceAfterRoundedPurchase.java)                                                                                                                                               |                                                                                                                                   |
-| 2810 | [Faulty Keyboard](https://leetcode.com/problems/faulty-keyboard)                                                                                                       |                                                                                   [![Java](assets/java.png)](src/FaultyKeyboard.java)                                                                                                                                               |                                                                                                                                   |
+| 2810 | [Faulty Keyboard](https://leetcode.com/problems/faulty-keyboard)                                                                                                       |                                                                                   [![Java](assets/java.png)](src/FaultyKeyboard.java)                                                                                                                                               |                                                                                                                                   |
+| 2815 | [Max Pair Sum in an Array](https://leetcode.com/problems/max-pair-sum-in-an-array)                                                                                     |                                                                                   [![Java](assets/java.png)](src/MaxPairSumInAnArray.java)                                                                                                                                               |                                                                                                                                   |
+| 2824 | [Count Pairs Whose Sum is Less than Target](https://leetcode.com/problems/count-pairs-whose-sum-is-less-than-target)                                                   |                                                                                   [![Java](assets/java.png)](src/)                                                                                                                                               |                                                                                                                                   |
+| 2828 | [Check if a String Is an Acronym of Words](https://leetcode.com/problems/check-if-a-string-is-an-acronym-of-words)                                                     |                                                                                   [![Java](assets/java.png)](src/)                                                                                                                                               |                                                                                                                                   |

Diff for: src/MaxPairSumInAnArray.java

@@ -0,0 +1,27 @@
+// https://leetcode.com/problems/max-pair-sum-in-an-array
+// T: O(N)
+// S: O(1)
+
+public class MaxPairSumInAnArray {
+    public int maxSum(int[] nums) {
+        final int[] digitMaxSum = new int[9];
+        int maxPairSum = 0;
+        for (int element : nums) {
+            final int maxDigit = getMaxDigit(element);
+            if (digitMaxSum[maxDigit - 1] != 0) {
+                maxPairSum = Math.max(maxPairSum, digitMaxSum[maxDigit - 1] + element);
+            }
+            digitMaxSum[maxDigit - 1] = Math.max(digitMaxSum[maxDigit - 1], element);
+        }
+        return maxPairSum == 0 ? -1 : maxPairSum;
+    }
+
+    private int getMaxDigit(int number) {
+        final String string = number + "";
+        int maxDigit = 0;
+        for (int i = 0 ; i < string.length() ; i++) {
+            maxDigit = Math.max(maxDigit, string.charAt(i) - '0');
+        }
+        return maxDigit;
+    }
+}