Method LockingTree

java/1993-operations-on-tree.java:6–16

(int[] parent)

    private final Map<Integer, List<Integer>> child;

    public LockingTree(int[] parent) {
        this.parent = parent;
        locked = new int[parent.length];
        child = new HashMap<>();

        for (int i = 0; i < parent.length; i++) {
            child.putIfAbsent(parent[i], new ArrayList<>());
            child.putIfAbsent(i, new ArrayList<>());
            child.get(parent[i]).add(i);
        }
    }

    public boolean lock(int num, int user) {
        if (locked[num] <= 0) {