Solution2.cpp

/**
 * 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:
    vector<vector<int>> verticalOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if (!root) return ans;
        map<int, vector<int>> d;
        queue<pair<TreeNode*, int>> q{{{root, 0}}};
        while (!q.empty()) {
            for (int n = q.size(); n; --n) {
                auto p = q.front();
                q.pop();
                root = p.first;
                int offset = p.second;
                d[offset].push_back(root->val);
                if (root->left) q.push({root->left, offset - 1});
                if (root->right) q.push({root->right, offset + 1});
            }
        }
        for (auto& [_, v] : d) {
            ans.push_back(v);
        }
        return ans;
    }
};