[SeongA] WEEK 03 Solutions

combination-sum/delight010.swift

@@ -0,0 +1,24 @@
+class Solution {
+    func combinationSum(_ candidates: [Int], _ target: Int) -> [[Int]] {
+        var result: [[Int]] = []
+        var current: [Int] = []
+        findCombination(0, target, candidates, &current, &result)
+        return result
+    }
+
+    func findCombination(_ index: Int, _ target: Int, _ candidates: [Int], _ current: inout [Int], _ result: inout [[Int]]) {
+        if target == 0 {
+            result.append(current)
+            return
+        }
+
+        for i in index..<candidates.count {
+            if candidates[i] <= target {
+                current.append(candidates[i])
+                findCombination(i, target - candidates[i], candidates, &current, &result)
+                current.removeLast()
+            }
+        }
+    }
+}
+

decode-ways/delight010.swift

@@ -0,0 +1,19 @@
+class Solution {
+    func numDecodings(_ s: String) -> Int {
+        var array = Array(s)
+        var dp: [Int: Int] = [array.count: 1]
+        for i in stride(from: array.count - 1, to: -1, by: -1) {
+            if array[i] == "0" {
+                dp[i] = 0
+            } else {
+                dp[i] = dp[i + 1]
+            }
+
+            if i + 1 < array.count && (array[i] == "1" || array[i] == "2" && "0123456".contains(array[i + 1])) {
+                dp[i, default: 0] += dp[i + 2] ?? 0
+            }
+        }
+        return dp[0]!
+    }
+}
+

maximum-subarray/delight010.swift

@@ -0,0 +1,12 @@
+class Solution {
+    func maxSubArray(_ nums: [Int]) -> Int {
+        var maxSum = nums[0]
+        var maxEndingHere = nums[0]
+        for i in 1..<nums.count {
+            maxEndingHere = max(nums[i], maxEndingHere + nums[i])
+            maxSum = max(maxSum, maxEndingHere)
+        }
+        return maxSum
+    }
+}
+

number-of-1-bits/delight010.swift

@@ -0,0 +1,14 @@
+class Solution {
+    func hammingWeight(_ n: Int) -> Int {
+        var number = n
+        var answer = 0
+        while number > 0 {
+            if number & 1 == 1 {
+                answer += 1
+            }
+            number = number >> 1
+        }
+        return answer
+    }
+}
+

valid-palindrome/delight010.swift

@@ -0,0 +1,29 @@
+class Solution {
+    func isPalindrome(_ s: String) -> Bool {
+        let string = s.replacingOccurrences(of: " ", with: "").lowercased().map { $0 }
+        var leftIndex = string.startIndex
+        var rightIndex = string.endIndex - 1
+        let letterRange: ClosedRange<UInt8> = 97...122
+        let numberRange: ClosedRange<UInt8> = 48...57
+        while leftIndex <= rightIndex {
+            guard let leftCharAscii = string[leftIndex].asciiValue else { break }
+            guard let rightCharAscii = string[rightIndex].asciiValue else { break }
+            if !letterRange.contains(leftCharAscii) && !numberRange.contains(leftCharAscii) {
+                leftIndex += 1
+                continue
+            }
+            if !letterRange.contains(rightCharAscii) && !numberRange.contains(rightCharAscii) {
+                rightIndex -= 1
+                continue
+            }
+            if leftCharAscii == rightCharAscii {
+                leftIndex += 1
+                rightIndex -= 1
+            } else {
+                return false
+            }
+        }
+        return true
+    }
+}
+