[DP] Add a solution to Different Ways to Add Parentheses

Author: Yi Gu

DP/DifferentWaysAddParentheses.swift

@@ -0,0 +1,50 @@
+/**
+ * Question Link: https://leetcode.com/problems/different-ways-to-add-parentheses/
+ * Primary idea: Divide and Conquer, calculate left and right separately and union results
+ *
+ * Note: Keep a memo dictionary to avoid duplicate calculations
+ * Time Complexity: O(n^n), Space Complexity: O(n)
+ *
+ */
+
+class DifferentWaysAddParentheses {
+    var memo = [String: [Int]]()
+
+    func diffWaysToCompute(_ input: String) -> [Int] {
+        if let nums = memo[input] {
+            return nums
+        }
+    
+        var res = [Int]()
+        let chars = Array(input.characters)
+        
+        for (i, char) in chars.enumerated() {
+            if char == "+" || char == "*" || char == "-" {
+                let leftResults = diffWaysToCompute(String(chars[0 ..< i]))
+                let rightResults = diffWaysToCompute(String(chars[i + 1 ..< chars.count]))
+                
+                for leftRes in leftResults {
+                    for rightRes in rightResults {
+                        if char == "+" {
+                            res.append(leftRes + rightRes)
+                        }
+                        if char == "-" {
+                            res.append(leftRes - rightRes)
+                        }
+                        if char == "*" {
+                            res.append(leftRes * rightRes)
+                        }
+                    }
+                }
+            }
+        }
+        
+        if res.count == 0 {
+            res.append(Int(input)!)
+        }
+        
+        memo[input] = res
+        
+        return res
+    }
+}