feat: solve No.1759

Author: BaffinLee

1701-1800/1759. Count Number of Homogenous Substrings.md

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+# 1759. Count Number of Homogenous Substrings
+
+- Difficulty: Medium.
+- Related Topics: Math, String.
+- Similar Questions: Consecutive Characters, Number of Substrings With Only 1s, Sum of Subarray Ranges, Count the Number of Good Subarrays.
+
+## Problem
+
+Given a string `s`, return **the number of **homogenous** substrings of **`s`**.** Since the answer may be too large, return it **modulo** `109 + 7`.
+
+A string is **homogenous** if all the characters of the string are the same.
+
+A **substring** is a contiguous sequence of characters within a string.
+
+ 
+Example 1:
+
+```
+Input: s = "abbcccaa"
+Output: 13
+Explanation: The homogenous substrings are listed as below:
+"a"   appears 3 times.
+"aa"  appears 1 time.
+"b"   appears 2 times.
+"bb"  appears 1 time.
+"c"   appears 3 times.
+"cc"  appears 2 times.
+"ccc" appears 1 time.
+3 + 1 + 2 + 1 + 3 + 2 + 1 = 13.
+```
+
+Example 2:
+
+```
+Input: s = "xy"
+Output: 2
+Explanation: The homogenous substrings are "x" and "y".
+```
+
+Example 3:
+
+```
+Input: s = "zzzzz"
+Output: 15
+```
+
+ 
+**Constraints:**
+
+
+	
+- `1 <= s.length <= 105`
+	
+- `s` consists of lowercase letters.
+
+
+## Solution
+
+```javascript
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var countHomogenous = function(s) {
+    var startIndex = 0;
+    var sum = 0;
+    var mod = Math.pow(10, 9) + 7;
+    for (var i = 0; i < s.length; i++) {
+        if (s[i] === s[i + 1]) continue;
+        var num = i - startIndex + 1;
+        sum += ((1 + num) * num / 2) % mod;
+        sum %= mod;
+        startIndex = i + 1;
+    }
+    return sum;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).