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| 1 | +# [1291. Sequential Digits (Medium)](https://leetcode.com/problems/sequential-digits/) |
| 2 | + |
| 3 | +<p>An integer has <em>sequential digits</em> if and only if each digit in the number is one more than the previous digit.</p> |
| 4 | + |
| 5 | +<p>Return a <strong>sorted</strong> list of all the integers in the range <code>[low, high]</code> inclusive that have sequential digits.</p> |
| 6 | + |
| 7 | +<p> </p> |
| 8 | +<p><strong>Example 1:</strong></p> |
| 9 | +<pre><strong>Input:</strong> low = 100, high = 300 |
| 10 | +<strong>Output:</strong> [123,234] |
| 11 | +</pre><p><strong>Example 2:</strong></p> |
| 12 | +<pre><strong>Input:</strong> low = 1000, high = 13000 |
| 13 | +<strong>Output:</strong> [1234,2345,3456,4567,5678,6789,12345] |
| 14 | +</pre> |
| 15 | +<p> </p> |
| 16 | +<p><strong>Constraints:</strong></p> |
| 17 | + |
| 18 | +<ul> |
| 19 | + <li><code>10 <= low <= high <= 10^9</code></li> |
| 20 | +</ul> |
| 21 | + |
| 22 | + |
| 23 | +**Related Topics**: |
| 24 | +[Backtracking](https://leetcode.com/tag/backtracking/) |
| 25 | + |
| 26 | +## Solution 1. |
| 27 | + |
| 28 | +```cpp |
| 29 | +// OJ: https://leetcode.com/problems/sequential-digits/ |
| 30 | +// Author: github.com/lzl124631x |
| 31 | +// Time: O(1) since there are limited number of valid integers |
| 32 | +// Space: O(1) |
| 33 | +class Solution { |
| 34 | + long get(int start, int len) { |
| 35 | + long ans = 0; |
| 36 | + while (len-- > 0) { |
| 37 | + ans = ans * 10 + start; |
| 38 | + ++start; |
| 39 | + } |
| 40 | + return ans; |
| 41 | + } |
| 42 | +public: |
| 43 | + vector<int> sequentialDigits(int low, int high) { |
| 44 | + vector<int> ans; |
| 45 | + int len = 0, tmp = len; |
| 46 | + while (tmp) { |
| 47 | + tmp /= 10; |
| 48 | + ++len; |
| 49 | + } |
| 50 | + while (len <= 9) { |
| 51 | + for (int i = 1; i <= 9 - len + 1; ++i) { |
| 52 | + long long n = get(i, len); |
| 53 | + if (n < low) continue; |
| 54 | + if (n > high) return ans; |
| 55 | + ans.push_back(n); |
| 56 | + } |
| 57 | + ++len; |
| 58 | + } |
| 59 | + return ans; |
| 60 | + } |
| 61 | +}; |
| 62 | +``` |
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