1291. Sequential Digits - Explanation

class Solution:
    def sequentialDigits(self, low: int, high: int) -> List[int]:
        res = []

        for num in range(low, high + 1):
            s = str(num)
            flag = True
            for i in range(1, len(s)):
                if ord(s[i]) - ord(s[i - 1]) != 1:
                    flag = False
                    break
            if flag:
                res.append(num)

        return res

class Solution:
    def sequentialDigits(self, low: int, high: int) -> List[int]:
        res = []
        low_digit, high_digit = len(str(low)), len(str(high))

        for digits in range(low_digit, high_digit + 1):
            for start in range(1, 10):
                if start + digits > 10:
                    break
                num = start
                prev = start
                for i in range(digits - 1):
                    num = num * 10
                    prev += 1
                    num += prev
                if low <= num <= high:
                    res.append(num)
        return res

class Solution:
    def sequentialDigits(self, low: int, high: int) -> List[int]:
        res = []
        queue = deque(range(1, 10))

        while queue:
            n = queue.popleft()
            if n > high:
                continue
            if low <= n <= high:
                res.append(n)
            ones = n % 10
            if ones < 9:
                queue.append(n * 10 + (ones + 1))

        return res

class Solution:
    def sequentialDigits(self, low: int, high: int) -> List[int]:
        res = []

        def dfs(num):
            if num > high:
                return
            if low <= num <= high:
                res.append(num)
            last_digit = num % 10
            if last_digit < 9:
                dfs(num * 10 + (last_digit + 1))

        for i in range(1, 10):
            dfs(i)

        return sorted(res)

class Solution:
    def sequentialDigits(self, low: int, high: int) -> List[int]:
        nums = "123456789"
        res = []
        for d in range(2, 10):
            for i in range(9 - d + 1):
                num = int(nums[i: i + d])
                if num > high:
                    break
                if low <= num <= high:
                    res.append(num)
        return res