2022.6.26-----leetcode.710

    //将[0,n-m-1]的黑名单数与[n−m,n-1]的白名单数进行映射，随机返回[0，n-m-1]里的数
    Map<Integer, Integer> b2w;
    Random random;
    int bound;
    public Solution(int n, int[] blacklist) {
        b2w = new HashMap<Integer, Integer>();
        random = new Random();
        int m = blacklist.length;
        bound = n - m;
        Set<Integer> black = new HashSet<Integer>();
        for (int b : blacklist) {
            if (b >= bound) {
                black.add(b);
            }
        }

        int w = bound;
        for (int b : blacklist) {
            if (b < bound) {
                while (black.contains(w)) {
                    ++w;
                }
                b2w.put(b, w);
                ++w;
            }
        }
    }

    public int pick() {
        int x = random.nextInt(bound);
        return b2w.getOrDefault(x, x);
    }