LeetCode----Unique Binary Search Trees 2

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         3     3      2      1

    \       /     /      / \      \

     3     2     1      1   3      2

    /     /       \                 \

   2     1         2                 3

分析：

递归即可。枚举所有的 root 节点，root 的左子树和右子树就是两个子问题，递归求解。

但是有一点要注意，容易出错。

如果子树的节点个数为零，这时候返回的应该是一个包含 NULL 的 vector;

/**

 * 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 *> generateTrees(int n) {

        vector<TreeNode*> res;

        if(n == 0){

            res.push_back(NULL);

            return res;

        }

        return generateTrees(1, n);

    }

private:

    vector<TreeNode *> generateTrees(int left, int right)

    {

        TreeNode *node(NULL);

        vector<TreeNode*> trees, left_tree, right_tree;

        if(left > right)

        {

            trees.push_back(NULL);

            return trees;

        }

        for(int i=left; i<=right; ++i)

        {

            left_tree = generateTrees(left, i-1);

            right_tree = generateTrees(i+1, right);

            for(auto j : left_tree)

              for(auto k : right_tree)

              {

                  node = new TreeNode(i);

                  node->left = j;

                  node->right = k;

                  trees.push_back(node);

              }

        }

        return trees;

    }

};

巴特西

LeetCode----Unique Binary Search Trees 2

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