1 public class MinFreeProblemSolver {
 2     public static int solve(int[] numbers) {
 3         boolean[] occurrence = new boolean[numbers.length];
 4         for (int number : numbers) {
 5             if (number < numbers.length) {
 6                 occurrence[number] = true;
 7             }
 8         }
 9         for (int i = 0; i < numbers.length; i++) {
10             if (!occurrence[i]) {
11                 return i;
12             }
13         }
14         return numbers.length;
15     }
16 }

 1 public class MinFreeProblemSolver {
 2     public static int solve(int[] numbers) {
 3         BitSet occurrence = new BitSet(numbers.length);
 4         for (int number : numbers) {
 5             if (number < numbers.length) {
 6                 occurrence.set(number);
 7             }
 8         }
 9         return occurrence.nextClearBit(0);
10     }
11 }

 1 public class MinFreeProblemSolver {
 2     public static int solve(int[] numbers) {
 3         return solve(numbers, 0, numbers.length - 1);
 4     }
 5
 6     private static int solve(int[] numbers, int fromIndex, int endIndex) {
 7         if (fromIndex == endIndex) {
 8             return numbers[fromIndex] == fromIndex ? fromIndex + 1 : fromIndex;
 9         }
10         int pivot = (fromIndex + endIndex) >>> 1;
11         int left = doPartition(numbers, fromIndex, endIndex, pivot);
12         if (left == pivot + 1) {
13             return solve(numbers, pivot + 1, endIndex);
14         } else {
15             return solve(numbers, fromIndex, pivot);
16         }
17     }
18
19     private static int doPartition(int[] numbers, int fromIndex, int endIndex, int pivot) {
20         int left = fromIndex;
21         for (int right = fromIndex; right <= endIndex; right++) {
22             if (numbers[right] <= pivot) {
23                 int temp = numbers[right];
24                 numbers[right] = numbers[left];
25                 numbers[left++] = temp;
26             }
27         }
28         return left;
29     }
30 }