95-GenerateTrees.py

# Python3

from typing import List

# Definition for a binary tree node.
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
        
class Solution:
    def generateTrees(self, n: int) -> List[TreeNode]:
        def generateTrees(start, end):
            if start > end:
                return [None,]
            
            allTrees = []
            for i in range(start, end + 1): # 枚举可行根节点
                leftTrees = generateTrees(start, i - 1)
                rightTrees = generateTrees(i + 1, end)

                for left in leftTrees:
                    for right in rightTrees:
                        currTree = TreeNode(i)
                        currTree.left = left
                        currTree.right = right
                        allTrees.append(currTree)

            return allTrees

        return generateTrees(1, n) if n else []

# C++ 
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

class Solution {
public:
    vector<TreeNode*> generateTrees(int start, int end) {
        if (start > end) {
            return { nullptr };
        }
        vector<TreeNode*> allTrees;
        // 枚举可行根节点
        for (int i = start; i <= end; i++) {
            // 获得所有可行的左子树集合
            vector<TreeNode*> leftTrees = generateTrees(start, i - 1);

            // 获得所有可行的右子树集合
            vector<TreeNode*> rightTrees = generateTrees(i + 1, end);

            // 从左子树集合中选出一棵左子树，从右子树集合中选出一棵右子树，拼接到根节点上
            for (auto& left : leftTrees) {
                for (auto& right : rightTrees) {
                    TreeNode* currTree = new TreeNode(i);
                    currTree->left = left;
                    currTree->right = right;
                    allTrees.emplace_back(currTree);
                }
            }
        }
        return allTrees;
    }

    vector<TreeNode*> generateTrees(int n) {
        if (!n) {
            return {};
        }
        return generateTrees(1, n);
    }
};