617. Merge Two Binary Trees

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还是递归遍历，只不过同时遍历两个tree，结束条件是其中一个树遍历到叶子了，这时候接下来的subtree就可以直接拿另外一个树的来用。

class Solution:
    def mergeTrees(self, t1: TreeNode, t2: TreeNode) -> TreeNode:
        if not t1: return t2
        if not t2: return t1
        t1.val += t2.val
        t1.left = self.mergeTrees(t1.left, t2.left)
        t1.right = self.mergeTrees(t1.right, t2.right)
        return t1