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  • 95. Unique Binary Search Trees II 不同bst的个数

    Given an integer n, return all the structurally unique BST's (binary search trees), which has exactly n nodes of unique values from 1 to n. Return the answer in any order.

     

    Example 1:

    Input: n = 3
    Output: [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]]
    

    Example 2:

    Input: n = 1
    Output: [[1]]
    

     

    Constraints:

    • 1 <= n <= 8

    复习时不会的地方:返回的树其实就是个list
    List<TreeNode> result = new ArrayList<TreeNode>();在子函数中,因为left新生成的树也是子树

    就是生成东西的题,用dc

    参考:https://leetcode.com/problems/unique-binary-search-trees-ii/discuss/31600/Quite-clean-Java-solution-with-explanation

    public class Solution {
        public List<TreeNode> generateTrees(int n) {
            if(n == 0)
                return new ArrayList<TreeNode>();
            return generateTrees(1, n);
        }
        
        List<TreeNode> generateTrees(int start, int end) {
            List<TreeNode> result = new ArrayList<TreeNode>();
            if(start > end) {
                result.add(null);
                return result;
            }
            for(int i = start; i <= end; i++) {
                List<TreeNode> leftSubTrees = generateTrees(start, i - 1);
                List<TreeNode> rightSubTrees = generateTrees(i + 1, end);
           //左右每个点都遍历到
    for(TreeNode left : leftSubTrees) { for(TreeNode right : rightSubTrees) { TreeNode root = new TreeNode(i); root.left = left; root.right = right; result.add(root); } } } return result; } }

     

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  • 原文地址:https://www.cnblogs.com/immiao0319/p/14939737.html
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