merging trees: iterative using stack, immutable trees

merge_bst.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode result = new TreeNode(0);
        
        if (t1 == null && t2 == null)
            return null;        
        if (t1 == null)
            return t2;
        if (t2 == null)
            return t1;
        
        stack.push(result);
        stack.push(t1);
        stack.push(t2);
        
        while (!stack.isEmpty()) {
            TreeNode p2 = stack.pop();
            TreeNode p1 = stack.pop();
            TreeNode p = stack.pop();
            
            p.val = p1.val + p2.val;    
            if (p1.left != null || p2.left != null) {
                if (p1.left == null) {
                    p.left = p2.left;
                } else if (p2.left == null) {
                    p.left = p1.left;
                } else {
                    TreeNode left = new TreeNode(0);
                    p.left = left;
                    stack.push(p.left);
                    stack.push(p1.left);
                    stack.push(p2.left);
                }
            }
            if (p1.right != null || p2.right != null) {
                if (p1.right == null) {
                    p.right = p2.right;
                } else if (p2.right == null) {
                    p.right = p1.right;
                } else {
                    TreeNode right = new TreeNode(0);
                    p.right = right;
                    stack.push(p.right);
                    stack.push(p1.right);
                    stack.push(p2.right);
                }
            }
        }
        
        return result;
    }
}