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/**
 * 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 {
private:
    vector<vector<int>> solution;
public:
    int getHeight(TreeNode *root) {
        // return -1 for null nodes
        if (!root)
            return -1;

        // first calculate the height of the left and right children
        int leftHeight = getHeight(root->left);
        int rightHeight = getHeight(root->right);

        // based on the height of the left and right children, obtain the height of the current (parent) node
        int currHeight = max(leftHeight, rightHeight) + 1;

        // create space for node located at `currHeight` if not already exists
        if(solution.size() == currHeight)
            solution.push_back({});

        // insert the value at the correct position in the solution array
        solution[currHeight].push_back(root->val);

        // return the height of the current node
        return currHeight;
    }

    vector<vector<int>> findLeaves(TreeNode* root) {
        solution.clear();
        getHeight(root);
        return solution;
    }
};