95 Unique Binary Search Trees II

https://leetcode.com/problems/unique-binary-search-trees-ii/#/description

Given an integern, generate all structurally uniqueBST's(binary search trees) that store values 1...n.

For example,
Givenn= 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

分析及代码

public List<TreeNode> generateTrees(int n) {
    if(n < 1) {
        return new ArrayList<TreeNode>();
    }
    return generate(1, n);
}

private List<TreeNode> generate(int start, int end) {
    List<TreeNode> list = new ArrayList<>();
    if(start > end) {
        list.add(null);
        return list;
    }
    if(start == end) {
        list.add(new TreeNode(start));
        return list;
    }
    for(int i = start; i <= end; i++) {
        List<TreeNode> left = generate(start, i - 1);
        List<TreeNode> right = generate(i + 1, end);
        for(TreeNode leftSubtree: left) {
            for(TreeNode rightSubtree: right) {
                TreeNode root = new TreeNode(i);
                root.left = leftSubtree;
                root.right = rightSubtree;
                list.add(root);
            }
        }
    }
    return list;
}

只能这样暴力解

要注意的是，在generate()函数里面, if(start > end), list还是要add(null);