15 人解决，35 人已尝试。
18 份提交通过，共有 203 份提交。
6.8 EMB 奖励。
单点时限: 2.0 sec
内存限制: 256 MB
A binary tree can either be empty or consist of one vertex, with two trees linked to it. These two trees are called a left and a right subtree. In each vertex there is one letter from the English alphabet. The vertex which is not a subtree of any vertex, is called a root. We say that a tree is a Binary Search Tree (BST) if for each vertex the following condition is satisfied: all letters in the left subtree precede in alphabetical order the letter in the root, and all letters from the right subtree follow the letter in the root. A code of a BST is:
either an empty string (containing 0 letters) when the tree is empty
or a string beginning with the letter from the root, followed by the code of the left subtree, followed by the code of the right subtree.
Let us consider all k-vertex BSTs containing in their vertices k initial letters of the English alphabet. Suppose we have a list of these codes written in alphabetical order. (n,k)-code is the n-th code on this list.
Example
There are exactly fourteen 4-vertex BSTs. These are (in alphabetical order):
abcd abdc acbd adbc adcb bacd badc cabd cbad dabc dacb dbac dcab dcba
The string badc is the (7,4)-code and it corresponds to the BST printed below:
Task
Write a program which:
1.reads the numbers n and k,
2.finds the (n, k)-code,
3.writes it out.
In the first and only line there are exactly two positive integers n and k, separated by a single space, 1<=k<=19. The number n is not greater than the number of codes of BST with k vertices.
In the first and only line there should be exactly one word (written in small letters) being the (n,k)-code.
11 4
dacb
15 人解决，35 人已尝试。
18 份提交通过，共有 203 份提交。
6.8 EMB 奖励。