java: add «643. Maximum Average Subarray I»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/maximum-average-subarray-i.java

@@ -0,0 +1,21 @@
+class Solution {
+  private long sumFirst(int[] nums, int k) {
+    long sum = 0;
+    for (var i = 0; i < k; ++i) {
+      sum += nums[i];
+    }
+    return sum;
+  }
+
+  public double findMaxAverage(int[] nums, int k) {
+    var runningSum = sumFirst(nums, k);
+    var foundMax = runningSum;
+
+    for (var i = k; i < nums.length; ++i) {
+      runningSum += nums[i] - nums[i - k];
+      foundMax = Math.max(foundMax, runningSum);
+    }
+
+    return (double) foundMax / k;
+  }
+}