863. All Nodes Distance K in Binary Tree

BFS + hashmap

Code

class Solution {
    void markParents(TreeNode* root, unordered_map<TreeNode*, TreeNode*>& parent_track, TreeNode* target)
    {
        queue<TreeNode*> queue;
        queue.push(root);
        while (!queue.empty()) {
            TreeNode* current = queue.front();
            queue.pop();
            if (current->left) {
                parent_track[current->left] = current;
                queue.push(current->left);
            }
            if (current->right) {
                parent_track[current->right] = current;
                queue.push(current->right);
            }
        }
    }

public:
    vector<int> distanceK(TreeNode* root, TreeNode* target, int k)
    {
        unordered_map<TreeNode*, TreeNode*> parent_track; // node -> parent
        markParents(root, parent_track, target);

        unordered_map<TreeNode*, bool> visited;
        queue<TreeNode*> queue;
        queue.push(target);
        visited[target] = true;
        int curr_level = 0;
        while (!queue.empty()) {
            int size = queue.size();
            if (curr_level++ == k)
                break;
            for (int i = 0; i < size; i++) {
                TreeNode* current = queue.front();
                queue.pop();
                if (current->left && !visited[current->left]) {
                    queue.push(current->left);
                    visited[current->left] = true;
                }
                if (current->right && !visited[current->right]) {
                    queue.push(current->right);
                    visited[current->right] = true;
                }
                if (parent_track[current] && !visited[parent_track[current]]) {
                    queue.push(parent_track[current]);
                    visited[parent_track[current]] = true;
                }
            }
        }
        vector<int> result;
        while (!queue.empty()) {
            TreeNode* current = queue.front();
            queue.pop();
            result.push_back(current->val);
        }
        return result;
    }
};