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865. Smallest Subtree with all the Deepest Nodes 👍

  • Time: $O(n)$
  • Space: $O(n)$
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struct T {
  TreeNode* lca;
  int depth;
};

class Solution {
 public:
  TreeNode* subtreeWithAllDeepest(TreeNode* root) {
    return dfs(root).lca;
  }

 private:
  T dfs(TreeNode* root) {
    if (root == nullptr)
      return {nullptr, 0};

    const T left = dfs(root->left);
    const T right = dfs(root->right);
    if (left.depth > right.depth)
      return {left.lca, left.depth + 1};
    if (left.depth < right.depth)
      return {right.lca, right.depth + 1};
    return {root, left.depth + 1};
  }
};

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class Solution {
  public TreeNode subtreeWithAllDeepest(TreeNode root) {
    return dfs(root).lca;
  }

  private record T(TreeNode lca, int depth) {}

  private T dfs(TreeNode root) {
    if (root == null)
      return new T(null, 0);

    T left = dfs(root.left);
    T right = dfs(root.right);
    if (left.depth > right.depth)
      return new T(left.lca, left.depth + 1);
    if (left.depth < right.depth)
      return new T(right.lca, right.depth + 1);
    return new T(root, left.depth + 1);
  }
}