leetcode971

 class Solution:
     def flipMatchVoyage(self, root, voyage):
         res = []
         self.i = 0
         def dfs(root):
             if not root: return True
             if root.val != voyage[self.i]: return False
             self.i += 1
             if root.left and root.left.val != voyage[self.i]:
                 res.append(root.val)
                 root.left,root.right = root.right, root.left
             return dfs(root.left) and dfs(root.right)
         return res if dfs(root) else [-1]

参考：https://leetcode.com/problems/flip-binary-tree-to-match-preorder-traversal/discuss/214216/JavaC%2B%2BPython-DFS-Solution