Find a Safe Walk Through a Grid — C++

// Time:  O(m * n)
// Space: O(m * n)

// 0-1 bfs, deque
class Solution {
public:
    bool findSafeWalk(vector<vector<int>>& grid, int health) {
        static const vector<pair<int, int>> directions = {{0, 1}, {0, -1},
                                                          {1, 0}, {-1, 0}};
        const pair<int, int> b = {0, 0}, t = {size(grid) - 1, size(grid[0]) - 1};
        if (!(0 + grid[0][0] < health)) {
            return false;
        }
        deque<pair<pair<int, int>, int>> dq = {{b, grid[0][0]}};
        unordered_set<int> lookup;
        while (!empty(dq)) {
            const auto [b, d] = dq.front(); dq.pop_front();
            if (b == t) {
                return true;
            }
            if (lookup.count(b.first * size(grid[0]) + b.second)) {
                continue;
            }
            lookup.emplace(b.first * size(grid[0]) + b.second);
            for (const auto& [dr, dc] : directions) {
                const auto& nb = make_pair(b.first + dr, b.second + dc);
                if (!(0 <= nb.first && nb.first < size(grid) &&
                      0 <= nb.second && nb.second < size(grid[0]) &&
                      !lookup.count(nb.first * size(grid[0]) + nb.second))) {
                    continue;
                }
                if (!grid[nb.first][nb.second]) {
                    dq.emplace_front(nb, d);
                } else if (d + 1 < health) {
                    dq.emplace_back(nb, d + 1);
                }
            }
        }
        return false;
    }
};