feat(leetcode): add No.2815

Author: mickey0524

2815.Max-Pair-Sum-in-an-Array.java

@@ -0,0 +1,48 @@
+// https://leetcode.com/problems/max-pair-sum-in-an-array/description/
+// algorithms
+// Easy (57.85%)
+// Total Accepted: 30.2K
+// Total Submissions: 52.2K
+
+
+class Solution {
+
+    public int maxSum(int[] nums) {
+        Map<Integer, List<Integer>> map = new HashMap<>();
+
+        for (int n : nums) {
+            int maxDigit = getMaxDigit(n);
+
+            List<Integer> list = map.get(maxDigit);
+            if (list == null) {
+                list = new ArrayList<>();
+                map.put(maxDigit, list);
+            }
+
+            list.add(n);
+        }    
+
+        int res = -1;
+        for (Map.Entry<Integer, List<Integer>> entry : map.entrySet()) {
+            List<Integer> list = entry.getValue();
+            int size = list.size();
+            if (size > 1) {
+                Collections.sort(list);
+                res = Math.max(res, list.get(size - 1) + list.get(size - 2));
+            }
+        }
+
+        return res;
+    }
+
+    private int getMaxDigit(int n) {
+        int maxDigit = -1;
+        while (n > 0) {
+            maxDigit = Math.max(maxDigit, n % 10);
+            n = n / 10;
+        }
+
+        return maxDigit;
+    }
+
+}