OD机试考题 - 数组二叉树（Java 2025 A卷 200分）

    //数组二叉树
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        //输入：2 1 3 4 5 6 7 -1 -1 -1 -1 -1 -1 -1 -1
        String[] str = sc.nextLine().split(" ");
        int[] tree = new int[str.length];
        for (int i = 0; i < str.length; i++) {
            tree[i] = Integer.parseInt(str[i]);
        }
        List<Integer> list = route(tree,0);
        //倒序排列
        Collections.reverse(list);
        //输出：2，1，4 
        System.out.println(list.toString().replace("[","").replace("]",""));
    }

    private static List<Integer> route(int[] tree,int index) {
        if (tree[index]==-1){
            return null;
        }
        List<Integer> treeleft = route(tree,2*index+1);
        List<Integer> treeright = route(tree,2*index+2);
        List<Integer> list = new ArrayList<>();
        //左右为空，返回自身
        if (treeleft==null&&treeright==null){
            list.add(tree[index]);
            return list;
        }
        //左空，返回右+自身
        else if (treeleft==null) {
            treeright.add(tree[index]);
            return treeright;
        }
        //右空，返回左+自身
        else if (treeright==null){
            treeleft.add(tree[index]);
        }else {
            //左，右节点不为-1时进行比较,拿最开始插入（叶节点）比较那个小
            if (treeleft.get(0) < treeright.get(0)) {
                treeleft.add(tree[index]);
                return treeleft;
            } else {
                treeright.add(tree[index]);
                return treeright;
            }
        }
        return null;
    }