#include <cassert>
#include <map>
#include <optional>

#ifdef _MF_TEST
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) {}
};
#endif

class Solution {
    void find_bottom_left(std::map<int, std::optional<int>> &view, int y,
                          TreeNode *node) {
        if (node == nullptr) {
            return;
        }

        auto &leftmost = view[y];
        if (!leftmost.has_value()) {
            leftmost = std::make_optional(node->val);
        }

        find_bottom_left(view, y - 1, node->left);
        find_bottom_left(view, y - 1, node->right);
    }

  public:
    int findBottomLeftValue(TreeNode *root) {
        assert(root != nullptr);

        std::map<int, std::optional<int>> leftview;

        find_bottom_left(leftview, 0, root);
        return leftview.begin()->second.value();
    }
};