sequential-digits.cpp

// Time:  O((8 + 1) * 8 / 2) = O(1)
// Space: O(8) = O(1)

class Solution {
public:
    vector<int> sequentialDigits(int low, int high) {
        vector<int> result;
        queue<int> q{{1, 2, 3, 4, 5, 6, 7, 8}};
        while (!q.empty()) {
            const auto num = q.front(); q.pop();
            if (num > high) {
                continue;
            }
            if (low <= num) {
                result.emplace_back(num);
            }
            if (num % 10 + 1 < 10) {
                q.emplace(num * 10 + num % 10 + 1);
            }
        }
        return result;
    }
};