LeetCode-95. Unique Binary Search Trees II
Description:
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
和第I题的区别是这题要返回所有的结果,不是结果的数目。所以要递归枚举:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution { public List<TreeNode> generate(int start, int end) {
List<TreeNode> ret = new ArrayList<TreeNode>(); if(start > end) {
ret.add(null);
return ret;
} for(int i=start; i<=end; i++) {
List<TreeNode> lTree = generate(start, i-1); //生成左子树集合
List<TreeNode> rTree = generate(i+1, end); //生成右子树集合
for(int j=0; j<lTree.size(); j++) {
for(int k=0; k<rTree.size(); k++) {
TreeNode node = new TreeNode(i+1); //连接到根节点
node.left = lTree.get(j);
node.right = rTree.get(k);
ret.add(node);
}
}
}
return ret;
} public List<TreeNode> generateTrees(int n) {
if(n <= 0) return new ArrayList<TreeNode>();
return generate(0, n-1);
}
}
C++:
/**
* 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 *> generate(int start, int end)
{
vector<TreeNode* > ret;
if (start > end)
{
ret.push_back(NULL);
return ret;
} for(int i = start; i <= end; i++)
{
vector<TreeNode* > lTree = generate(start, i - );
vector<TreeNode* > rTree = generate(i + , end);
for(int j = ; j < lTree.size(); j++)
for(int k = ; k < rTree.size(); k++)
{
TreeNode *node = new TreeNode(i + );
ret.push_back(node);
node->left = lTree[j];
node->right = rTree[k];
}
} return ret;
} vector<TreeNode *> generateTrees(int n) {
vector<TreeNode* > ret;
if(n <= ) return ret;
return generate(, n - );
}
};
同样的代码同样的测试数据Java代码为什么效率高这么多,难道是跑代码的服务器不一样?还是Java后台调用代码写的好?
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