feat: Add solution for LeetCode problem 647

Author: WhiteHyun

palindromic-substrings/WhiteHyun.swift

@@ -0,0 +1,105 @@
+//  
+//  647. Palindromic Substrings
+//  https://leetcode.com/problems/palindromic-substrings/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/07/14.
+//  
+
+class Solution {
+
+  // dp way
+  func countSubstringsUsingDP(_ s: String) -> Int {
+    let array = Array(s)
+    let n = array.count
+    var count = 0
+
+    var dp: [[Bool]] = .init(
+      repeating: .init(repeating: false, count: n),
+      count: n
+    )
+
+    for i in dp.indices {
+      dp[i][i] = true
+      count += 1
+    }
+
+    for i in dp.indices.dropLast() where array[i] == array[i + 1] {
+      dp[i][i + 1] = true
+      count += 1
+    }
+
+    for length in stride(from: 3, through: n, by: 1) {
+      for i in 0 ... n - length where array[i] == array[i + length - 1] && dp[i + 1][i + length - 2] {
+        dp[i][i + length - 1] = true
+        count += 1
+      }
+    }
+
+    return count
+  }
+
+  // center and expand way
+  func countSubstrings(_ s: String) -> Int {
+    let array = Array(s)
+    var count = 0
+
+    for startIndex in array.indices {
+      let evenCount = expandAroundCenter(array, startIndex, startIndex + 1)
+      let oddCount = expandAroundCenter(array, startIndex, startIndex)
+      count += evenCount + oddCount
+    }
+
+    return count
+  }
+
+  private func expandAroundCenter(_ array: [Character], _ left: Int, _ right: Int) -> Int {
+    var count = 0
+    var l = left
+    var r = right
+    while l >= 0 && r < array.count && array[l] == array[r] {
+      l -= 1
+      r += 1
+      count += 1
+    }
+
+    return count
+  }
+
+  
+  // Brute Force
+  func countSubstringsUsingBruteForce(_ s: String) -> Int {
+    let array = Array(s)
+    var count = 0
+
+    for startIndex in array.indices {
+      for targetIndex in startIndex ..< array.count where array[startIndex] == array[targetIndex] {
+        if isPalindrome(
+          array,
+          startIndex,
+          targetIndex
+        ) {
+          count += 1
+        }
+      }
+    }
+
+    return count
+  }
+
+  private func isPalindrome(_ array: [Character], _ start: Int, _ end: Int) -> Bool {
+    var startIndex = start
+    var endIndex = end
+    while startIndex < endIndex {
+      if array[startIndex] == array[endIndex] {
+        startIndex += 1
+        endIndex -= 1
+        continue
+      }
+
+      return false
+    }
+
+    return true
+  }
+}