time-taken-to-cross-the-door.cpp

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

// queue, simulation
class Solution {
public:
    vector<int> timeTaken(vector<int>& arrival, vector<int>& state) {
        enum {UNKNOWN = -1, ENTERING, EXITING};
        vector<int> result(size(arrival));
        int curr = numeric_limits<int>::min(), direction = UNKNOWN;
        vector<queue<int>> q(2);
        const auto& go_until = [&](int t) {
            while (curr <= t && any_of(cbegin(q), cend(q), [](const auto& x) { return !empty(x); })) {
                if (empty(q[direction])) {
                    direction ^= 1;
                }
                result[q[direction].front()] = curr++;
                q[direction].pop();
            }
        };

        for (int i = 0; i < size(arrival); ++i) {
            go_until(arrival[i] - 1);
            q[state[i]].emplace(i);
            if (!(arrival[i] <= curr)) {
                curr = arrival[i], direction = EXITING;
            }
        }
        go_until(numeric_limits<int>::max());
        return result;
    }
};