smallest number in an array java code example

Example 1: find the largest and smallest number in an unsorted integer array in Java

public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);
    System.out.println("Number of integers to enter:");
    int numberEnter = keyboard.nextInt();
    int numbers[] = new int[numberEnter];
    int pos = 0;
    do {
        System.out.printf("Please enter integer #%d/%d:%n", pos, numberEnter);
        numbers[pos++] = keyboard.nextInt();
    } while (pos < numberEnter && keyboard.hasNextInt());
    int min = numbers[0];
    int max = numbers[0];
    for (pos = 1; pos < numbers.length; pos++) {
        if (numbers[pos] < min) { // <-- test min.
            min = numbers[pos];
        }
        if (numbers[pos] > max) { // <-- test max.
            max = numbers[pos];
        }
    }
    // Display everything.
    System.out.printf("%s Min: %d Max: %d%n", Arrays.toString(numbers),
            min, max);
}

Example 2: smallest positive integer not in array java

If the expected running time should be linear, you can't use a TreeSet, which sorts the input and therefore requires O(NlogN). Therefore you should use a HashSet, which requires O(N) time to add N elements.

Besides, you don't need 4 loops. It's sufficient to add all the positive input elements to a HashSet (first loop) and then find the first positive integer not in that Set (second loop).

int N = A.length;
Set<Integer> set = new HashSet<>();
for (int a : A) {
    if (a > 0) {
        set.add(a);
    }
}
for (int i = 1; i <= N + 1; i++) {
    if (!set.contains(i)) {
        return i;
    }
}