218_Longest_Arithmetic_Subsequence_of_Given_Difference

Author: partho-maple

README.md

@@ -780,7 +780,8 @@ BFS, DFS, Dijkstra, Floyd–Warshall, Bellman-Ford, Kruskal, Prim's, Minimum Spa
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leetcode.com/swift/DS_ALGO_PRAC.playground/Sources/swift/218_Longest_Arithmetic_Subsequence_of_Given_Difference.swift

@@ -0,0 +1,42 @@
+// Brute force DFS. Wrong anserr for some cases
+class Solution {
+    func longestSubsequence(_ arr: [Int], _ difference: Int) -> Int {
+        guard arr.count > 1 else {
+            return 1
+        }
+        var maxLength = 1
+        for i in 0..<arr.count {
+            longestSubsequenceDFSHelper(arr, difference, i, 1, &maxLength)
+        }
+        return maxLength
+    }
+
+    func longestSubsequenceDFSHelper(_ arr: [Int], _ difference: Int, _ currentStartIndex: Int, _ currentLength: Int, _ maxLength: inout Int) {
+        if currentStartIndex >= arr.count - 1 {
+            maxLength = [maxLength, currentLength].max()!
+        }
+        for i in currentStartIndex..<arr.count {
+            let currentDiff = arr[i] - arr[currentStartIndex]
+            if currentDiff == difference {
+                longestSubsequenceDFSHelper(arr, difference, i, currentLength + 1, &maxLength)
+            }
+        }
+    }
+}
+
+// DP, Accepted. Time O(n)
+class Solution {
+    func longestSubsequence(_ arr: [Int], _ difference: Int) -> Int {
+        guard arr.count > 1 else {
+            return 1
+        }
+        var (maxLength, dp) = (1, [Int:Int]()) // arr itemas key and length till i as value
+        dp[arr.first!, default: 0] += 1
+        for i in 1..<arr.count {
+            let pre = arr[i] - difference
+            dp[arr[i], default: 0] = 1 + (dp[pre] ?? 0)
+            maxLength = [maxLength, dp[arr[i]]!].max()!
+        }
+        return maxLength
+    }
+}