[LeetCode] Add January 18.

Author: pin3da

solved/LeetCode/Challenges/2020/January/18.kt

@@ -0,0 +1,16 @@
+class Solution {
+  fun maxOperations(nums: IntArray, k: Int): Int {
+    val freq = mutableMapOf<Int, Int>()
+    var ans = 0
+    for (i in nums) {
+      val target = k - i
+      if ((freq[target] ?: 0) > 0) {
+        freq.put(target, freq[target]!! -1)
+        ans++
+      } else {
+        freq[i] = (freq[i] ?: 0) + 1 
+      }
+    }
+    return ans
+  }
+}