feat: solve No.1930

Author: BaffinLee

1901-2000/1930. Unique Length-3 Palindromic Subsequences.md

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+# 1930. Unique Length-3 Palindromic Subsequences
+
+- Difficulty: Medium.
+- Related Topics: Hash Table, String, Prefix Sum.
+- Similar Questions: Count Palindromic Subsequences.
+
+## Problem
+
+Given a string `s`, return **the number of **unique palindromes of length three** that are a **subsequence** of **`s`.
+
+Note that even if there are multiple ways to obtain the same subsequence, it is still only counted **once**.
+
+A **palindrome** is a string that reads the same forwards and backwards.
+
+A **subsequence** of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.
+
+
+	
+- For example, `"ace"` is a subsequence of `"abcde"`.
+
+
+ 
+Example 1:
+
+```
+Input: s = "aabca"
+Output: 3
+Explanation: The 3 palindromic subsequences of length 3 are:
+- "aba" (subsequence of "aabca")
+- "aaa" (subsequence of "aabca")
+- "aca" (subsequence of "aabca")
+```
+
+Example 2:
+
+```
+Input: s = "adc"
+Output: 0
+Explanation: There are no palindromic subsequences of length 3 in "adc".
+```
+
+Example 3:
+
+```
+Input: s = "bbcbaba"
+Output: 4
+Explanation: The 4 palindromic subsequences of length 3 are:
+- "bbb" (subsequence of "bbcbaba")
+- "bcb" (subsequence of "bbcbaba")
+- "bab" (subsequence of "bbcbaba")
+- "aba" (subsequence of "bbcbaba")
+```
+
+ 
+**Constraints:**
+
+
+	
+- `3 <= s.length <= 105`
+	
+- `s` consists of only lowercase English letters.
+
+
+
+## Solution
+
+```javascript
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var countPalindromicSubsequence = function(s) {
+    var a = 'a'.charCodeAt(0);
+    var res = 0;
+    for (var i = 0; i < 26; i++) {
+        var char = String.fromCharCode(i + a);
+        var first = s.indexOf(char);
+        var last = s.lastIndexOf(char);
+        if (last - first < 2) continue;
+        var map = {};
+        for (var j = first + 1; j < last; j++) {
+            if (!map[s[j]]) {
+                res += 1;
+                map[s[j]] = true;
+            }
+        }
+    }
+    return res;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).