95. Unique Binary Search Trees II

分成两边， left right排列组合加到root， lo==hi就返回当前值

 

 

class Solution {
    public List<TreeNode> generateTrees(int n) {
        if(n == 0) return (new ArrayList<>());
        return generate(1, n);
        
    }
    
    public List<TreeNode> generate(int lo, int hi) {
        List<TreeNode> list = new ArrayList<>();
        if(lo > hi) {
            list.add(null);
            return list;
        }
        if(lo == hi) {
            list.add(new TreeNode(lo));
            return list;
        }
        
        for(int i = lo; i <= hi; i++) {
            List<TreeNode> left = generate(lo, i-1);
            List<TreeNode> right = generate(i+1, hi);
            
            for(TreeNode lNode: left) {
                for(TreeNode rNode: right) {
                    TreeNode root = new TreeNode(i);
                    root.left = lNode;
                    root.right = rNode;
                    list.add(root);
                }
            }
            
        }
        return list;
    }
}