【Leetcode】Unique Binary Search Trees II

题目链接：https://leetcode.com/problems/unique-binary-search-trees-ii/

题目：

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

思路：

当根结点分别为1~n时，递归遍历，根结点大于左子树小于右子树。

算法：

public List<TreeNode> generateTrees(int n) {
if (n == 0)
return null;
return dsp(1, n);
}

public List<TreeNode> dsp(int start, int end) {
List<TreeNode> res = new ArrayList<TreeNode>();
if (start > end) {
res.add(null);
return res;
}

for (int i = start; i <= end; i++) {
List<TreeNode> left = dsp(start, i - 1);
List<TreeNode> right = dsp(i + 1, end);
for (TreeNode l : left) {
for (TreeNode r : right) {
TreeNode root = new TreeNode(i);
root.left = l;
root.right = r;
res.add(root);
}
}
}
return res;
}