Java Solution, 16ms, beat 100%


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    Recursive thinking is very simple, not repeat them. The key to reducing time complexity is to use the original TreeNode node instead of creating a new TreeNode object.

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

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