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1469. Find All The Lonely Nodes 👍

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

    dfs(root, false, ans);

    return ans;
  }

 private:
  void dfs(TreeNode* root, bool isLonely, vector<int>& ans) {
    if (root == nullptr)
      return;
    if (isLonely)
      ans.push_back(root->val);

    dfs(root->left, root->right == nullptr, ans);
    dfs(root->right, root->left == nullptr, ans);
  }
};

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class Solution {
  public List<Integer> getLonelyNodes(TreeNode root) {
    List<Integer> ans = new ArrayList<>();

    dfs(root, false, ans);

    return ans;
  }

  private void dfs(TreeNode root, boolean isLonely, List<Integer> ans) {
    if (root == null)
      return;
    if (isLonely)
      ans.add(root.val);

    dfs(root.left, root.right == null, ans);
    dfs(root.right, root.left == null, ans);
  }
}