Jan_9(865)

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* subtreeWithAllDeepest(TreeNode* root) {
        if (!root) return nullptr;

        unordered_map<TreeNode*, TreeNode*> parent;
        queue<TreeNode*> q;
        q.push(root);
        parent[root] = nullptr;

        vector<TreeNode*> lastLevel;

        // BFS traversal
        while (!q.empty()) {
            int size = q.size();
            lastLevel.clear();

            for (int i = 0; i < size; i++) {
                TreeNode* node = q.front();
                q.pop();
                lastLevel.push_back(node);

                if (node->left) {
                    parent[node->left] = node;
                    q.push(node->left);
                }
                if (node->right) {
                    parent[node->right] = node;
                    q.push(node->right);
                }
            }
        }

        unordered_set<TreeNode*> deepest(lastLevel.begin(), lastLevel.end());

        // Move upward until one node remains
        while (deepest.size() > 1) {
            unordered_set<TreeNode*> next;
            for (auto node : deepest) {
                next.insert(parent[node]);
            }
            deepest = next;
        }

        return *deepest.begin();
    }
};