Vk Cup 2017 B

Main2.java

import javafx.util.Pair;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.*;
import java.util.function.ToIntFunction;

public class Main2 {

    public static void main(String[] args) throws FileNotFoundException {

//        try (Scanner scanner = new Scanner(new FileInputStream("3"))) {
        try (Scanner scanner = new Scanner(System.in)) {
            PriorityQueue<Node> nodes = new PriorityQueue<>((o1, o2) -> {

                int compare = Integer.compare(o1.weight, o2.weight);
                if (compare != 0) {
                    return  compare;
                }
                return - Integer.compare(o1.index, o2.index);
            });
            int n = scanner.nextInt();
            Node p = null;
            for (int i = 0; i < n; i++) {
                int weight = scanner.nextInt();
                if (i == 0 && weight == 0) {
                    System.out.println(-1);
                    return;
                }
                if (i == 0) {
                    p = new Node(i + 1, weight);
                } else {
                    nodes.add(new Node(i + 1, weight));

                }
            }
//            System.out.println("n=" + n);
            List<Pair<Integer, Integer>> order = new ArrayList<>();
            while (!nodes.isEmpty()) {
                Node node = nodes.remove();

                Node parent = findNodeMinWeight(nodes, p);
                if (parent == null) {
                    System.out.println(-1);
                    return;
                }
//                System.out.println(parent.index + " send " + node.index);
                parent.weight--;
                order.add(new Pair<>(parent.index, node.index));
                if (parent.index == 1 && nodes.size() == 0) {
                    break;
                }

            }
            System.out.println(order.size());
            for (int i = order.size() - 1; i >= 0; i--) {
                Pair<Integer, Integer> pair = order.get(i);
                System.out.println(pair.getKey() + " " + pair.getValue());
            }
        }
    }

    public static Node findNodeMinWeight(PriorityQueue<Node> nodes, Node p) {
        if (nodes.size() == 0) {
            return p;
        }
        Iterator<Node> it = nodes.iterator();
        while (it.hasNext()) {
            Node node = it.next();
            if (node.weight > 0 ) {
                return node;

            }
        }
        if (p.weight > 1) {
            return p;
        }
        return null;
    }


    public static class Node {
        int index;
        int weight;


        public Node(int index, int weight) {
            this.index = index;
            this.weight = weight;
        }

        @Override
        public String toString() {
            return "{" + index +
                    ", " + weight +
                    '}';
        }
    }
}