Merge pull request soapyigu#197 from soapyigu/DP

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

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
+ }
+}