Leetcode 971. Flip Binary Tree To Match Preorder Traversal

Given a binary tree and a target preorder sequence voyage, find the minimum set of nodes to flip (swap their left and right children) so the tree's preorder equals voyage, or return [-1] if impossible. Algorithmically, simulate a preorder traversal and greedily flip a node when its left child's value doesn't match the next voyage value, recording flipped nodes.