java: add «881. Boats to Save People»

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

java/boats-to-save-people.java

@@ -0,0 +1,24 @@
+import java.util.Arrays;
+
+class Solution {
+  public int numRescueBoats(int[] people, int limit) {
+    Arrays.parallelSort(people);
+
+    int counter = 0;
+
+    int left = 0, right = people.length - 1;
+    while (left <= right && left < people.length && right >= 0) {
+      ++counter;
+
+      // they both fit
+      if (left < right && people[left] + people[right] <= limit) {
+        ++left;
+      }
+
+      // the heaviest person is always included
+      --right;
+    }
+
+    return counter;
+  }
+}