/**
 * 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) {
        if (t1 == null) {
            return t2;
        }
        if (t2 == null) {
            return t1;
        }
        // 先合并根节点
        t1.val += t2.val;
        // 再递归合并左右子树
        // 左节点合并
        t1.left = mergeTrees(t1.left, t2.left);
        // 右节点合并
        t1.right = mergeTrees(t1.right, t2.right);
        // 返回合并后的新二叉树
        return t1;
    }
}