test.java

/**
 * 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 TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null){
            return null;
        }
        TreeNode root = new TreeNode();
        if (root1 == null){
            root.val = root2.val;
        }
        else if (root2 == null){
            root.val = root1.val;
        }
        else{
            root.val = root1.val + root2.val;
        }
        if (root1.left != null && root2.left != null){
            root.left = mergeTrees(root1.left, root2.left);
        }
        else if (root1.left != null && root2.left == null){
            root.left = mergeTrees(root1.left, null);
        }
        else if (root1.left == null && root2.left != null){
            root.left = mergeTrees(null, root2.left);
        }
        else{
            root.left = mergeTrees(null, null);
        }
        if (root1.right != null && root2.right != null){
            root.right = mergeTrees(root1.right, root2.right);
        }
        else if (root1.right != null && root2.right == null){
            root.right = mergeTrees(root1.right, null);
        }
        else if (root1.right == null && root2.right != null){
            root.right = mergeTrees(null, root2.right);;
        }
        else{
            root.right = mergeTrees(null, null);
        }
        
        return root; 
    }
}