95. 不同的二叉搜索树 II (Java)
给定一个整数 n,生成所有由 1 ... n 为节点所组成的 二叉搜索树 。
示例:
输入:3
输出:
[
[1,null,3,2],
[3,2,null,1],
[3,1,null,null,2],
[2,1,3],
[1,null,2,null,3]
]
解释:
以上的输出对应以下 5 种不同结构的二叉搜索树:
1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3
提示:
0 <= n <= 8
/** * 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 generateSubTrees(1,n); } public List<TreeNode> generateSubTrees(int start, int end) { if(start == end) { List<TreeNode> rootList = new ArrayList<>(); TreeNode node = new TreeNode (start); rootList.add(node); return rootList; } List<TreeNode> rootList = new ArrayList<>(); for (int mid = start; mid <= end; mid++){ List<TreeNode> leftList = new ArrayList<>(); List<TreeNode> rightList = new ArrayList<>(); if(mid > start) leftList = generateSubTrees(start, mid-1); if(mid < end) rightList = generateSubTrees(mid+1, end); //只有左子树的情况 if(rightList.isEmpty()) { for(int i = 0; i < leftList.size(); i++){ TreeNode root = new TreeNode(mid, leftList.get(i), null); rootList.add(root); } } //只有右子树的情况 else if(leftList.isEmpty()) { for(int j = 0; j < rightList.size(); j++){ TreeNode root = new TreeNode(mid, null, rightList.get(j)); rootList.add(root); } } //左右子树都有的情况 else { for(int i = 0; i < leftList.size(); i++) { for(int j = 0; j < rightList.size(); j++){ TreeNode root = new TreeNode(mid, leftList.get(i), rightList.get(j)); rootList.add(root); } } } } return rootList; } }