19.2.24 [LeetCode 95] Unique Binary Search Trees II

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

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

   1         3     3      2      1
           /     /      /       
     3     2     1      1   3      2
    /     /                        
   2     1         2                 3

题意

给定一数n，求1-n所组成的所有可能的BST树的集合

题解

class Solution {
public:
    vector<TreeNode*> build(int s, int e) {
        if (s > e)return vector<TreeNode*>();
        vector<TreeNode*>ans;
        if (s == e) {
            TreeNode*tmp = new TreeNode(s);
            ans.push_back(tmp);
            return ans;
        }
        for (int i = s; i <= e; i++) {
            vector<TreeNode*>left = build(s, i - 1), right = build(i + 1, e);
            for(int j=0;j<left.size();j++)
                for (int k = 0; k < right.size(); k++) {
                    TreeNode*tmp = new TreeNode(i);
                    tmp->left = left[j], tmp->right = right[k];
                    ans.push_back(tmp);
                }
            if (left.size() == 0 || right.size() == 0) {
                for (int j = 0; j < left.size(); j++) {
                    TreeNode*tmp = new TreeNode(i);
                    tmp->left = left[j];
                    ans.push_back(tmp);
                }
                for (int j = 0; j < right.size(); j++) {
                    TreeNode*tmp = new TreeNode(i);
                    tmp->right = right[j];
                    ans.push_back(tmp);
                }
            }
        }
        return ans;
    }
    vector<TreeNode*> generateTrees(int n) {
        return build(1, n);
    }
};