All Elements in Two Binary Search Trees

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

Example 1:

Input: root1 = [2,1,4], root2 = [1,0,3]
Output: [0,1,1,2,3,4]

Example 2:

Input: root1 = [1,null,8], root2 = [8,1]
Output: [1,1,8,8]

Constraints:

• The number of nodes in each tree is in the range [0, 5000].

• -105 <= Node.val <= 105

Solution:

/**
 * 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) {
        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        
        List<Integer> result = new ArrayList<>();
        
        while(root1!=null || root2!=null || !s1.empty() || !s2.empty()){
            while(root1!=null)
            {
                s1.push(root1);
                root1=root1.left;
            }
            while(root2!=null)
            {
                s2.push(root2);
                root2=root2.left;
            }
            if(s2.empty() || (!s1.empty() && s1.peek().val <= s2.peek().val))
            {
                root1 = s1.pop();
                result.add(root1.val);
                root1 = root1.right;
            }
            else
                {
                root2 = s2.pop();
                result.add(root2.val);
                root2 = root2.right;
            }
            
        }
        
        return result;
    }
}