95. Unique Binary Search Trees II

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LeetCode

link
Approach: Recursion

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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 {
    private List<TreeNode> genTreeList(int start,int end){
        List<TreeNode> ls=new ArrayList<>();
        if(start>end) ls.add(null);
        for(int i=start;i<=end;i++){
            List<TreeNode> leftTreeList=genTreeList(start,i-1);
            List<TreeNode> rightTreeList=genTreeList(i+1,end);
            for(TreeNode left:leftTreeList){
                for(TreeNode right:rightTreeList){
                    TreeNode root=new TreeNode(i);
                    root.left=left;
                    root.right=right;
                    ls.add(root);
                }
            }
        }
        return ls;
    }
    public List<TreeNode> generateTrees(int n) {
        if(n==0) return new ArrayList<>();
        else return genTreeList(1,n);
    }
}

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