leetcode617. Merge Two Binary Trees

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合并二叉树，如果其中一颗树为空，直接返回另一棵；否则递归构造二叉树节点即可。

 public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null) return t2;
        if (t2 == null) return t1;
        TreeNode root = new TreeNode(t1.val + t2.val);
        root.left = mergeTrees(t1.left, t2.left);
        root.right = mergeTrees(t1.right, t2.right);
        return root;
    }