1305 All Elements in Two Binary Search Trees

Leetcode

Given two binary search trees root1 and root2, return a list containing all the integers from both trees sorted in ascending order.

先通过前序遍历把树转换为链表，然后合并链表。不确定这样做是最好的方法，可能还有更优的解法。

/**
 * 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<Integer> getAllElements(TreeNode root1, TreeNode root2) {
        LinkedList<Integer> l1 = new LinkedList<>();
        LinkedList<Integer> l2 = new LinkedList<>();
        
        if(root1 != null) preOrder(root1, l1);
        if(root2 != null) preOrder(root2, l2);
        
        List<Integer> ret = new LinkedList<>();
        
        while(l1.size()>0 && l2.size()>0){
            if(l1.peek()<l2.peek()){
                ret.add(l1.pop());
            } else {
                ret.add(l2.pop());
            }
        }
        
        if(l1.size()>0){
            ret.addAll(l1);
        } else {
            ret.addAll(l2);
        }
        
        return ret;
    }
    
    private void preOrder(TreeNode node, List<Integer> list){
        if(node.left!=null){
            preOrder(node.left, list);
        }
        list.add(node.val);
        if(node.right!=null){
            preOrder(node.right, list);
        }
    }
    
    
}