g1201_1300.s1206_design_skiplist.Skiplist Maven / Gradle / Ivy

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package g1201_1300.s1206_design_skiplist;

// #Hard #Design #Linked_List #2022_03_08_Time_14_ms_(96.71%)_Space_48_MB_(85.19%)

@SuppressWarnings("java:S2245")
public class Skiplist {
    private static final int INIT_CAPACITY = 8;
    private final int minBoundary;
    private final Node head;
    private int headCapacity;
    private int headLevel = 0;

    private static class Node {
        private final int val;
        private Node[] next;

        private Node(int val, int level) {
            this.val = val;
            next = new Node[level];
        }
    }

    public Skiplist() {
        this(INIT_CAPACITY);
    }

    public Skiplist(int size) {
        if (size == 0) {
            throw new IllegalArgumentException("size should be greater than 0");
        }
        if (size < INIT_CAPACITY) {
            size = INIT_CAPACITY;
        }
        minBoundary = size / 2;
        headCapacity = size;
        head = new Node(0, size);
    }

    public boolean search(int target) {
        Node curr = head;
        for (int i = headLevel - 1; i >= 0; i--) {
            while (curr.next[i] != null) {
                int cmp = target - curr.next[i].val;
                if (cmp < 0) {
                    break;
                } else if (cmp > 0) {
                    curr = curr.next[i];
                } else {
                    return true;
                }
            }
        }
        return false;
    }

    public void add(int num) {
        Node[] update = new Node[headLevel + 1];
        update[headLevel] = head;
        buildUpdate(num, update);
        int level = getRandomLevel();
        if (level > headLevel) {
            if (headLevel == headCapacity) {
                resizeHead(2 * headCapacity);
            }
            headLevel++;
        }
        Node x = new Node(num, level);
        for (int i = 0; i < level; i++) {
            Node n = update[i].next[i];
            update[i].next[i] = x;
            x.next[i] = n;
        }
    }

    public boolean erase(int num) {
        if (headLevel == 0) {
            return false;
        }
        Node[] update = new Node[headLevel];
        buildUpdate(num, update);
        if (update[0].next[0] == null || update[0].next[0].val != num) {
            return false;
        }
        for (int i = 0; i < headLevel; i++) {
            if (update[i].next[i] == null || update[i].next[i].val != num) {
                break;
            }
            update[i].next[i] = update[i].next[i].next[i];
        }
        if (head.next[headLevel - 1] == null
                && --headLevel >= minBoundary
                && headLevel == headCapacity / 4) {
            resizeHead(headCapacity / 2);
        }
        return true;
    }

    private void buildUpdate(int x, Node[] update) {
        Node curr = head;
        for (int i = headLevel - 1; i >= 0; i--) {
            while (curr.next[i] != null && curr.next[i].val < x) {
                curr = curr.next[i];
            }
            update[i] = curr;
        }
    }

    private int getRandomLevel() {
        int maxLevel = 14;
        int level = 1;
        int limit = Math.min(maxLevel, headLevel + 1);
        double p = 0.5;
        while (Math.random() < p && level < limit) {
            level++;
        }
        return level;
    }

    private void resizeHead(int size) {
        Node[] copy = new Node[size];
        System.arraycopy(head.next, 0, copy, 0, headLevel);
        head.next = copy;
        headCapacity = size;
    }
}