415. Add Strings

Author: TBXark

415. Add Strings.md

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+# 415. Add Strings
+
+### 2017-04-05
+
+Given two non-negative integers `num1` and `num2` represented as string, return the sum of `num1` and `num2`.
+
+**Note:**
+
+1. The length of both `num1` and `num2` is < 5100.
+2. Both `num1` and `num2` contains only digits `0-9`.
+3. Both `num1` and `num2` does not contain any leading zero.
+4. You **must not use any built-in BigInteger library** or **convert the inputs to integer** directly.
+
+
+
+# Solution
+
+```swift
+class Solution {
+    func addStrings(_ num1: String, _ num2: String) -> String {
+        func toIntArray(string: String) -> [Int8] {
+            guard var n = string.cString(using: String.Encoding.utf8) else { return [] }
+            n = n.reversed()
+            n.removeFirst()
+            return n.map({ (v: Int8) -> Int8 in
+                return v - 48
+            })
+        }
+        
+        let n1 = toIntArray(string: num1)
+        let n2 = toIntArray(string: num2)
+        let count = max(n1.count, n2.count)
+        var n3 = [Int8](repeating: 0, count: count + 1)
+
+        for i in 0..<count {
+            let c = (i < n1.count ? n1[i] : 0) + (i < n2.count ? n2[i] : 0)
+            n3[i] = n3[i] + c
+            if n3[i] >= 10 {
+                n3[i + 1] += n3[i] / 10
+                n3[i] = n3[i] % 10
+            }
+        }
+         if let n = n3.last, n == 0 {
+            n3.removeLast()
+        }
+        
+        if n3.count > 0 {
+            return n3.reduce("", { "\($1)\($0)"})
+        } else {
+            return "0"
+        }       
+    }
+}
+```
+