leetcode-95-不同的二叉搜索樹 II

題目鏈接 https://leetcode-cn.com/problems/unique-binary-search-trees-ii/submissions/

思路：

將1-n中的任意一個數視爲樹的根節點，左邊的視爲左子樹，右邊視爲右子樹。然後遞歸的對左右子樹進行這種操作。

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

    public List<TreeNode> generateTrees(int start, int end) {
         List<TreeNode> list = new ArrayList<>();

         if(start > end) {
             list.add(null);
             return  list;
         }
         for(int i = start; i <= end; i++) {
             List<TreeNode> left = generateTrees(start, i-1);
             List<TreeNode> right = generateTrees(i+1, end);

             for (TreeNode l : left) {
                 for (TreeNode r : right) {
                     TreeNode node = new TreeNode(i);
                     node.left = l;
                     node.right = r;
                     list.add(node);
                 }
             }
         }
         return list;
    }