Leetcode: Unique Binary Search Trees II

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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

这种问题，基本就是递归，关键是合适的递归函数。

/** * 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 *> result;        result = generateTreesUtil(1, n);                return result;    }        vector<TreeNode *> generateTreesUtil(int start, int end) {        vector<TreeNode *> result;        if (start > end) {            result.push_back(NULL);        }        else if (start == end) {            result.push_back(new TreeNode(start));        }        else {            for (int i = start; i <= end; ++i) {                vector<TreeNode *> left = generateTreesUtil(start, i - 1);                vector<TreeNode *> right = generateTreesUtil(i + 1, end);                for (int ll = 0; ll < left.size(); ++ll) {                    for (int rr = 0; rr < right.size(); ++rr) {                        TreeNode *root = new TreeNode(i);                        root->left = left[ll];                        root->right = right[rr];                        result.push_back(root);                    }                }            }        }                return result;    }};

====================第二次=======================

/** * 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) {        return getTrees(1, n + 1);    }        vector<TreeNode*> getTrees(int start, int end) {        vector<TreeNode*> result;        if (start >= end) {            result.push_back(NULL);        }        else {            for (int i = start; i < end; ++i) {                auto lefts = getTrees(start, i);                auto rights = getTrees(i + 1, end);                for (int ll = 0; ll < lefts.size(); ++ll) {                    for (int rr = 0; rr < rights.size(); ++rr) {                        TreeNode* root = new TreeNode(i);                        root->left = lefts[ll];                        root->right = rights[rr];                        result.push_back(root);                    }                }            }        }                return result;    }};

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