1291

Author: lzl124631x

leetcode/1291. Sequential Digits/README.md

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+# [1291. Sequential Digits (Medium)](https://leetcode.com/problems/sequential-digits/)
+
+<p>An&nbsp;integer has <em>sequential digits</em> if and only if each digit in the number is one more than the previous digit.</p>
+
+<p>Return a <strong>sorted</strong> list of all the integers&nbsp;in the range <code>[low, high]</code>&nbsp;inclusive that have sequential digits.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+<pre><strong>Input:</strong> low = 100, high = 300
+<strong>Output:</strong> [123,234]
+</pre><p><strong>Example 2:</strong></p>
+<pre><strong>Input:</strong> low = 1000, high = 13000
+<strong>Output:</strong> [1234,2345,3456,4567,5678,6789,12345]
+</pre>
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>10 &lt;= low &lt;= high &lt;= 10^9</code></li>
+</ul>
+
+
+**Related Topics**:  
+[Backtracking](https://leetcode.com/tag/backtracking/)
+
+## Solution 1.
+
+```cpp
+// OJ: https://leetcode.com/problems/sequential-digits/
+// Author: github.com/lzl124631x
+// Time: O(1) since there are limited number of valid integers
+// Space: O(1)
+class Solution {
+    long get(int start, int len) {
+        long ans = 0;
+        while (len-- > 0) {
+            ans = ans * 10 + start;
+            ++start;
+        }
+        return ans;
+    }
+public:
+    vector<int> sequentialDigits(int low, int high) {
+        vector<int> ans;
+        int len = 0, tmp = len;
+        while (tmp) {
+            tmp /= 10;
+            ++len;
+        }
+        while (len <= 9) {
+            for (int i = 1; i <= 9 - len + 1; ++i) {
+                long long n = get(i, len);
+                if (n < low) continue;
+                if (n > high) return ans;
+                ans.push_back(n);
+            }
+            ++len;
+        }
+        return ans;
+    }
+};
+```