[Array] Optimize the solution to Product of Array Except Self

[DP] Optimize code style to Unique Paths II

Author: soapyigu

Array/ProductExceptSelf.swift

@@ -2,45 +2,26 @@
  * Question Link: https://leetcode.com/problems/product-of-array-except-self/
  * Primary idea: Use two arrays to hold multiplication result from left and right sides 
  *               while iterating the original array
- * Time Complexity: O(n), Space Complexity: O(n)
+ * Time Complexity: O(n), Space Complexity: O(1)
  */
 
 class ProductExceptSelf {
-    func productExceptSelf(nums: [Int]) -> [Int] {
-        var res = [Int]()
-    
+    func productExceptSelf(_ nums: [Int]) -> [Int] {
+        var res = Array(repeating: 1, count: nums.count)
+        var right = 1
+        
         guard nums.count > 0 else {
             return res
         }
-        
-        let left = _initLeft(nums)
-        let right = _initRight(nums)
-        
-        for i in 0..<nums.count {
-            res.append(left[i] * right[i])
-        }
-        
-        return res
-    }
 
-    private func _initLeft(nums: [Int]) -> [Int] {
-        var left = [Int]()
-        left.append(1)
-        
         for i in 1..<nums.count {
-            left.append(left[i - 1] * nums[i - 1])
+            res[i] = res[i - 1] * nums[i - 1]
         }
-        
-        return left
-    }
-    
-    private func _initRight(nums: [Int]) -> [Int] {
-        var right = Array(count: nums.count, repeatedValue: 1)
-        
-        for i in (nums.count - 2).stride(through: 0, by: -1) {
-            right[i] = right[i + 1] * nums[i + 1]
+        for i in (0..<nums.count).reversed() {
+            res[i] = right * res[i]
+            right = right * nums[i]
         }
 
-        return right
+        return res
     }
 }

DP/UniquePathsII.swift

@@ -5,28 +5,37 @@
  */
 
 class UniquePathsII {
-    func uniquePathsWithObstacles(obstacleGrid: [[Int]]) -> Int {
+    func uniquePathsWithObstacles(_ obstacleGrid: [[Int]]) -> Int {
         let m = obstacleGrid.count
+        guard m > 0 else {
+            return 0
+        }
+        
         let n = obstacleGrid[0].count
+        guard n > 0 else {
+            return 0
+        }
 
-        var pathNums = Array(count: m, repeatedValue: Array(count: n, repeatedValue: 0))
-        return _helper(&pathNums, m - 1, n - 1, obstacleGrid)
+        var dp = Array(repeating: Array(repeating: -1, count: n), count: m)
+        
+        return help(m - 1, n - 1, &dp, obstacleGrid)
     }
 
-    func _helper(inout _ pathNums: [[Int]], _ m: Int, _ n: Int, _ obstacleGrid: [[Int]]) -> Int {
-        // termination
-        if m < 0 || n < 0 || obstacleGrid[m][n] == 1 {
+    fileprivate func help(_ m: Int, _ n: Int, _ dp: inout [[Int]], _ obstacleGrid: [[Int]]) -> Int {
+        if m < 0 || n < 0 {
+            return 0
+        }
+        if obstacleGrid[m][n] == 1 {
             return 0
         }
         if m == 0 && n == 0 {
             return 1
         }
-        if pathNums[m][n] != 0 {
-            return pathNums[m][n]
+        if dp[m][n] != -1 {
+            return dp[m][n]
         }
 
-        pathNums[m][n] = _helper(&pathNums, m - 1, n, obstacleGrid) + _helper(&pathNums, m, n - 1, obstacleGrid)
-        
-        return pathNums[m][n]
+        dp[m][n] = help(m - 1, n, &dp, obstacleGrid) + help(m, n - 1, &dp, obstacleGrid)
+        return dp[m][n]
     }
 }

README.md

@@ -60,7 +60,7 @@
 [Minimum Size Subarray Sum](https://leetcode.com/problems/minimum-size-subarray-sum/)| [Swift](./Array/MinimumSizeSubarraySum.swift)| Medium| O(n)| O(1)|
 [Maximum Size Subarray Sum Equals k](https://leetcode.com/problems/maximum-size-subarray-sum-equals-k/)| [Swift](./Array/MaximumSizeSubarraySumEqualsK.swift)| Medium| O(n)| O(n)|
 [Smallest Range](https://leetcode.com/problems/smallest-range/)| [Swift](./Array/SmallestRange.swift)| Hard | O(nm)| O(nm)|
-[Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/)| [Swift](./Array/ProductExceptSelf.swift)| Medium| O(n)| O(n)|
+[Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/)| [Swift](./Array/ProductExceptSelf.swift)| Medium| O(n)| O(1)|
 [Rotate Array](https://leetcode.com/problems/rotate-array/)| [Swift](./Array/RotateArray.swift)| Easy| O(n)| O(1)|
 [Rotate Image](https://leetcode.com/problems/rotate-image/)| [Swift](./Array/RotateImage.swift)| Medium| O(n^2)| O(1)|
 [Spiral Matrix](https://leetcode.com/problems/spiral-matrix/)| [Swift](./Array/SpiralMatrix.swift)| Medium| O(n^2)| O(1)|