First Missing Positive Integer

/*
 * Problem : Process an array.
 * Input   : n, then n integers.
 * Output  : The missing number.
 * Approach: Cyclic placement.
 * Time    : O(n)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static int solve(int[] a) {
        // Step 1: The answer is between 1 and n + 1. So we only care about values in the range 1..n.
        int n = a.length;

        // Step 2: Put each value x in its own place: index x - 1. Swap until the value at i is wrong for every place or already placed.
        for (int i = 0; i < n; i++) {
            while (a[i] >= 1 && a[i] <= n && a[a[i] - 1] != a[i]) {
                int target = a[i] - 1;
                int temporary = a[target];
                a[target] = a[i];
                a[i] = temporary;
            }
        }

        // Step 3: The first index i where a[i] != i + 1 gives the answer i + 1.
        for (int i = 0; i < n; i++) {
            if (a[i] != i + 1) {
                return i + 1;
            }
        }
        return n + 1;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        System.out.println(solve(a));
    }
}