/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null)
            return t2;
        if (t2 == null)
            return t1;
        t1.val += t2.val;
        t1.left = mergeTrees(t1.left, t2.left);
        t1.right = mergeTrees(t1.right, t2.right);
        return t1;
    }

    // public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
    //     if (t1 == null)
    //         return t2;
    //     Stack < TreeNode[] > stack = new Stack < > ();
    //     stack.push(new TreeNode[] {t1, t2});
    //     while (!stack.isEmpty()) {
    //         TreeNode[] t = stack.pop();
    //         if (t[0] == null || t[1] == null) {
    //             continue;
    //         }
    //         t[0].val += t[1].val;
    //         if (t[0].left == null) {
    //             t[0].left = t[1].left;
    //         } else {
    //             stack.push(new TreeNode[] {t[0].left, t[1].left});
    //         }
    //         if (t[0].right == null) {
    //             t[0].right = t[1].right;
    //         } else {
    //             stack.push(new TreeNode[] {t[0].right, t[1].right});
    //         }
    //     }
    //     return t1;
    // }
}