Leetcode-95-不同的二叉搜索树Ⅱ

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题目

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

image-20200721092547034

递归

遍历1...n中的所有元素, 以当前元素为根结点. 计算所有可能的左子树和右子树, 并将所有左子树-右子树组合拼接到根结点上.

代码如下

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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) {
        if (n == 0) {
            return new ArrayList<TreeNode>();
        }
        return generateTrees(1, n);
    }

    public List<TreeNode> generateTrees(int start, int end) {
        ArrayList<TreeNode> roots = new ArrayList<>();
        if (start > end) {
            roots.add(null);
            return roots;
        }
        for (int i = start; i <= end; i++) {
            List<TreeNode> leftTrees = generateTrees(start, i - 1);  // 获取所有可能的左子树
            List<TreeNode> rightTrees = generateTrees(i + 1, end);  // 获取所有可能的右子树
            
            // 遍历所有左子树和右子树的组合, 拼接到根结点上
            for (TreeNode leftTree : leftTrees) {
                for (TreeNode rightTree : rightTrees) {
                    TreeNode root = new TreeNode(i, leftTree, rightTree);
                    roots.add(root);
                }
            }
        }
        return roots;
    }
}

image-20200721092906650