LeetCode: Unique Binary Search Trees II [096]

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<TreeNode *> getAllBinaryTrees(int start, int end){
        vector<TreeNode*> trees;
        if(start>end){              //注意这里没有等号，Unique Binary Search Trees只是计数，叶子节点和NULL节点都计数为1， 而本题就不一样了，叶子节点作为一棵独立的子树生成。
            trees.push_back(NULL);
            return trees;
        }
        
        for(int k=start; k<=end; k++){
            //得到左子树集合
            vector<TreeNode*>leftTrees = getAllBinaryTrees(start, k-1);
            //得到右子树集合
            vector<TreeNode*>rightTrees = getAllBinaryTrees(k+1, end);
            
            //组合，生成二叉搜索树
            for(int l=0; l<leftTrees.size(); l++){
                for(int r=0; r<rightTrees.size(); r++){
                    //创建根节点
                    TreeNode *root = (TreeNode*)malloc(sizeof(TreeNode));
                    root->val=k;
                    root->left=leftTrees[l];
                    root->right=rightTrees[r];
                    trees.push_back(root);
                }
            }
        }
        return trees;
    }

    vector<TreeNode *> generateTrees(int n) {
        vector<TreeNode*>result;
        if(n==0){result.push_back(NULL);return result;}      //当n==0时是一棵空树，别忘了
        result = getAllBinaryTrees(1, n);
        return result;
    }
};