Unique Binary Search Trees II

描述

Given n, generate all structurally unique BST's (binary search trees) that store values 1…n.

For example,Given n = 3, your program should return all 5 unique BST's shown below.

  1.    1         3     3      2      1
  2.     \       /     /      / \      \
  3.      3     2     1      1   3      2
  4.     /     /       \                 \
  5.    2     1         2                 3

分析

见前面一题。

代码

  1. // Unique Binary Search Trees II
  2. // 时间复杂度TODO，空间复杂度TODO
  3. class Solution {
  4. public:
  5.     vector<TreeNode *> generateTrees(int n) {
  6.         if (n == 0) return vector<TreeNode*>();
  7.         return generate(1, n);
  8.     }
  9. private:
  10.     vector<TreeNode *> generate(int start, int end) {
  11.         vector<TreeNode*> subTree;
  12.         if (start > end) {
  13.             subTree.push_back(nullptr);
  14.             return subTree;
  15.         }
  16.         for (int k = start; k <= end; k++) {
  17.             vector<TreeNode*> leftSubs = generate(start, k - 1);
  18.             vector<TreeNode*> rightSubs = generate(k + 1, end);
  19.             for (auto i : leftSubs) {
  20.                 for (auto j : rightSubs) {
  21.                     TreeNode *node = new TreeNode(k);
  22.                     node->left = i;
  23.                     node->right = j;
  24.                     subTree.push_back(node);
  25.                 }
  26.             }
  27.         }
  28.         return subTree;
  29.     }
  30. };

相关题目

原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/cpp/binary-tree/bst/unique-binary-search-trees-ii.html