kt: add «2342. Max Sum of a Pair With Equal Sum of Digits»

URL:	https://leetcode.com/problems/max-sum-of-a-pair-with-equal-sum-of-digits/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/max-sum-of-a-pair-with-equal-sum-of-digits.kt

@@ -0,0 +1,36 @@
+class Solution {
+    private fun digitSum(_num: Int): Int {
+        var sum = 0
+
+        var num = _num
+        while (num > 0) {
+            sum += num % 10
+            num /= 10
+        }
+
+        return sum
+    }
+
+    fun maximumSum(nums: IntArray): Int =
+        mutableMapOf<Int, Int>().let { best ->
+            nums.withIndex().maxOf { num ->
+                // find the digit sum
+                val s = digitSum(num.value)
+
+                // construct the sum of the maximum so far and current value
+                val bestIndex = best.getOrDefault(s, -1)
+                val currentSum =
+                    when (bestIndex) {
+                        -1 -> -1
+                        else -> num.value + nums[bestIndex]
+                    }
+
+                // if found bigger value, update cached index
+                if (bestIndex == -1 || num.value > nums[bestIndex]) {
+                    best[s] = num.index
+                }
+
+                currentSum
+            }
+        }
+}