feat: add No.1291

Author: BaffinLee

1201-1300/1291. Sequential Digits.md

@@ -0,0 +1,65 @@
+# 1291. Sequential Digits
+
+- Difficulty: Medium.
+- Related Topics: Enumeration.
+- Similar Questions: .
+
+## Problem
+
+An integer has **sequential digits** if and only if each digit in the number is one more than the previous digit.
+
+Return a **sorted** list of all the integers in the range `[low, high]` inclusive that have sequential digits.
+
+ 
+Example 1:
+```
+Input: low = 100, high = 300
+Output: [123,234]
+```Example 2:
+```
+Input: low = 1000, high = 13000
+Output: [1234,2345,3456,4567,5678,6789,12345]
+```
+ 
+**Constraints:**
+
+
+	
+- `10 <= low <= high <= 10^9`
+
+
+
+## Solution
+
+```javascript
+/**
+ * @param {number} low
+ * @param {number} high
+ * @return {number[]}
+ */
+var sequentialDigits = function(low, high) {
+    var len1 = low.toString().length;
+    var len2 = high.toString().length;
+    var res = [];
+    for (var i = len1; i <= len2; i++) {
+        for (var j = 1; j <= 9 - i + 1; j++) {
+            var num = Array(i).fill(0)
+                .map((_, index) => j + index)
+                .reduce((sum, n) => sum * 10 + n, 0);
+            if (num >= low && num <= high) {
+                res.push(num);
+            }
+        }
+    }
+    return res;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(log(n)).
+* Space complexity : O(log(n)).