/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone{
	public static void main(String[] args) {
        int[] a = new int[]{100, 2, 3, 1, 4, -10, 5, 10, 21, 6, 8, -2020, 2000000000, 2000000001};
        System.out.println(findFirstMissingPositive(a));
 
        a = new int[]{2, 3, 1, 4, 6, 5, 7};
        System.out.println(findFirstMissingPositive(a));
    }
 
    static int findFirstMissingPositive(int[] a) {
        int n = a.length;
        boolean[] was = new boolean[n];
 
        for (int v : a) {
            if (v > 0 && v <= n) {
                was[v - 1] = true;
            }
        }
 
        for (int i = 0; i < n; i++) {
            if (!was[i]) return i + 1;
        }
 
        return n + 1;
    }
}

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