[DP] Add a solution to Palindromic Substrings

DP/PalindromicSubstrings.swift

@@ -0,0 +1,47 @@
+/**
+ * Question Link: https://leetcode.com/problems/palindromic-substrings/
+ * Primary idea: 2D Dynamic Programming, update boolean array based on
+ *               current two characters' equity and the previous boolean subarray
+ * Time Complexity: O(n^2), Space Complexity: O(n^2)
+ *
+ */
+
+class PalindromicSubstrings {
+    func countSubstrings(_ s: String) -> Int {
+        var palinCount = 0, dp = Array(repeating: Array(repeating: false, count: s.count), count: s.count)
+        var s = Array(s)
+
+        // init case with distance of 0 and 1
+        for i in 0..<s.count {
+            dp[i][i] = true
+            palinCount += 1
+        }
+
+        guard s.count > 1 else {
+            return palinCount
+        }
+
+        for i in 0..<s.count - 1 {
+            if s[i] == s[i + 1] {
+                dp[i][i + 1] = true
+                palinCount += 1
+            }
+        }
+
+        guard s.count > 2 else {
+            return palinCount
+        }
+
+        for distance in 2...s.count - 1 {
+            for i in 0..<s.count - distance {
+                if s[i] == s[i + distance] && dp[i + 1][i + distance - 1] {
+                    dp[i][i + distance] = true
+                    palinCount += 1
+                }
+            }
+        }
+
+        return palinCount
+
+    }
+}

README.md

@@ -28,7 +28,7 @@
 * [Microsoft](#microsoft)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 249 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
+[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 250 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
 
 
 ## Array
@@ -192,6 +192,7 @@
 [Coin Change](https://leetcode.com/problems/coin-change/)| [Swift](./DP/CoinChange.swift)| Medium| O(n^2)| O(n)|
 [Coin Change II](https://leetcode.com/problems/coin-change-ii/)| [Swift](./DP/CoinChangeII.swift)| Medium| O(n^2)| O(n)|
 [Longest Increasing Subsequence](https://leetcode.com/problems/longest-increasing-subsequence/)| [Swift](./DP/LongestIncreasingSubsequence.swift)| Medium| O(n^2)| O(n)|
+[Palindromic Substrings](https://leetcode.com/problems/palindromic-substrings/)| [Swift](./DP/PalindromicSubstrings.swift)| Medium| O(n^2)| O(n^2)|
 [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/)| [Swift](./DP/LongestPalindromicSubstring.swift)| Medium| O(n^2)| O(n^2)|
 [Perfect Squares](https://leetcode.com/problems/perfect-squares/)| [Swift](./DP/PerfectSquares.swift)| Medium| O(n^2)| O(n)|
 [House Robber](https://leetcode.com/problems/house-robber/)| [Swift](./DP/HouseRobber.swift)| Easy| O(n)| O(1)|