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971. Flip Binary Tree To Match Preorder Traversal

  • Time: $O(n)$
  • Space: $O(h)$
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class Solution {
 public:
  vector<int> flipMatchVoyage(TreeNode* root, vector<int>& voyage) {
    vector<int> ans;

    dfs(root, 0, voyage, ans);

    return ans;
  }

 private:
  void dfs(TreeNode* root, int&& i, const vector<int>& voyage,
           vector<int>& ans) {
    if (!root)
      return;
    if (root->val != voyage[i++]) {
      ans.clear();
      ans.push_back(-1);
      return;
    }

    if (i < voyage.size() && root->left && root->left->val != voyage[i]) {
      // flip root
      ans.push_back(root->val);
      dfs(root->right, move(i), voyage, ans);
      dfs(root->left, move(i), voyage, ans);
    } else {
      dfs(root->left, move(i), voyage, ans);
      dfs(root->right, move(i), voyage, ans);
    }
  }
};
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class Solution {
  public List<Integer> flipMatchVoyage(TreeNode root, int[] voyage) {
    List<Integer> ans = new ArrayList<>();

    dfs(root, voyage, ans);

    return ans;
  }

  private int i = 0;

  private void dfs(TreeNode root, int[] voyage, List<Integer> ans) {
    if (root == null)
      return;
    if (root.val != voyage[i++]) {
      ans.clear();
      ans.add(-1);
      return;
    }

    if (i < voyage.length && root.left != null && root.left.val != voyage[i]) {
      // flip root
      ans.add(root.val);
      dfs(root.right, voyage, ans);
      dfs(root.left, voyage, ans);
    } else {
      dfs(root.left, voyage, ans);
      dfs(root.right, voyage, ans);
    }
  }
}