LC.P95[不同的二叉搜索树II]

方法一：回溯

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<TreeNode> generateTrees(int n) {
        return dfs(1, n);
    }

    private List<TreeNode> dfs(int left, int right) {
        List<TreeNode> ans = new ArrayList<>();
        if (left > right) {
            ans.add(null);
            return ans;
        }
        for (int i = left; i <= right; ++i) {
            for (TreeNode x : dfs(left, i - 1)) {
                for (TreeNode y : dfs(i + 1, right)) {
                    TreeNode node = new TreeNode(i);
                    node.left = x;
                    node.right = y;
                    ans.add(node);
                }
            }
        }
        return ans;
    }
}
  • 时间复杂度：卡特兰数
  • 空间复杂度：卡特兰数