【leetcode】1215.Stepping Numbers

题目如下：

A Stepping Number is an integer such that all of its adjacent digits have an absolute difference of exactly 1. For example, 321 is a Stepping Number while 421 is not.

Given two integers low and high, find and return a sorted list of all the Stepping Numbers in the range [low, high] inclusive.

Example 1:

Input: low = 0, high = 21
Output: [0,1,2,3,4,5,6,7,8,9,10,12,21]

Constraints:

• 0 <= low <= high <= 2 * 10^9

解题思路：如果x是一个Stepping Number，假设x的个位是y，那么x*10 + y - 1 (y-1 >=0) 和 x*10 + y + 1 (y+1<=9) 也是Stepping Number，根据这个规律把所有符合条件的数字求出来即可。

代码如下：

class Solution(object):
    def countSteppingNumbers(self, low, high):
        """
        :type low: int
        :type high: int
        :rtype: List[int]
        """
        queue = range(0,10)
        res = set()
        while len(queue) > 0:
            val = queue.pop(0)
            if val >= low and val <= high:
                res.add(val)
            last = int(str(val)[-1])
            if last < 9:
                new_val = int(str(val) + str(last+1))
                if new_val <= high:
                    queue.append(new_val)
            if last > 0:
                new_val = int(str(val) + str(last-1))
                if new_val <= high:
                    queue.append(new_val)
        return sorted(list(res))