Merge branch 'raywenderlich/master' into LinearRegression

Author: james-mux

Heap/Heap.swift

@@ -112,6 +112,8 @@ public struct Heap<T> {
    * larger than the old one; in a min-heap it should be smaller.
    */
   public mutating func replace(index i: Int, value: T) {
+    guard i < elements.count else { return }
+    
     assert(isOrderedBefore(value, elements[i]))
     elements[i] = value
     shiftUp(index: i)
@@ -141,6 +143,8 @@ public struct Heap<T> {
    * to know the node's index, which may actually take O(n) steps to find.
    */
   public mutating func removeAtIndex(i: Int) -> T? {
+    guard i < elements.count else { return nil }
+    
     let size = elements.count - 1
     if i != size {
       swap(&elements[i], &elements[size])

Heap/Tests/HeapTests.swift

@@ -196,6 +196,12 @@ class HeapTests: XCTestCase {
     XCTAssertTrue(verifyMaxHeap(h))
     XCTAssertEqual(h.elements, [100, 50, 70, 10, 20, 60, 65])
 
+    //test index out of bounds
+    let v = h.removeAtIndex(10)
+    XCTAssertEqual(v, nil)
+    XCTAssertTrue(verifyMaxHeap(h))
+    XCTAssertEqual(h.elements, [100, 50, 70, 10, 20, 60, 65])
+    
     let v1 = h.removeAtIndex(5)
     XCTAssertEqual(v1, 60)
     XCTAssertTrue(verifyMaxHeap(h))
@@ -308,5 +314,10 @@ class HeapTests: XCTestCase {
     h.replace(index: 5, value: 13)
     XCTAssertTrue(verifyMaxHeap(h))
     XCTAssertEqual(h.elements, [16, 14, 13, 8, 7, 10, 3, 2, 4, 1])
+    
+    //test index out of bounds
+    h.replace(index: 20, value: 2)
+    XCTAssertTrue(verifyMaxHeap(h))
+    XCTAssertEqual(h.elements, [16, 14, 13, 8, 7, 10, 3, 2, 4, 1])
   }
 }

Palindromes/Palindromes.playground/Contents.swift

@@ -0,0 +1,36 @@
+import Cocoa
+
+public func palindromeCheck (text: String?) -> Bool {
+  if let text = text {
+    let mutableText = text.stringByTrimmingCharactersInSet(.whitespaceCharacterSet()).lowercaseString
+    let length: Int = mutableText.characters.count
+    
+    guard length >= 1 else {
+      return false
+    }
+    
+    if length == 1 {
+      return true
+    } else if mutableText[mutableText.startIndex] == mutableText[mutableText.endIndex.predecessor()] {
+      let range = Range<String.Index>(mutableText.startIndex.successor()..<mutableText.endIndex.predecessor())
+      return palindromeCheck(mutableText.substringWithRange(range))
+    }
+  }
+  
+  return false
+}
+
+// Test to check that non-palindromes are handled correctly:
+palindromeCheck("owls")
+
+// Test to check that palindromes are accurately found (regardless of case and whitespace:
+palindromeCheck("lol")
+palindromeCheck("race car")
+palindromeCheck("Race fast Safe car")
+
+// Test to check that palindromes are found regardless of case:
+palindromeCheck("HelloLLEH")
+
+// Test that nil and empty Strings return false:
+palindromeCheck("")
+palindromeCheck(nil)

Palindromes/Palindromes.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='osx'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

Palindromes/Palindromes.playground/playground.xcworkspace/contents.xcworkspacedata

@@ -0,0 +1,21 @@
+import Cocoa
+
+public func palindromeCheck (text: String?) -> Bool {
+  if let text = text {
+    let mutableText = text.stringByTrimmingCharactersInSet(.whitespaceCharacterSet()).lowercaseString
+    let length: Int = mutableText.characters.count
+    
+    guard length >= 1 else {
+      return false
+    }
+    
+    if length == 1 {
+      return true
+    } else if mutableText[mutableText.startIndex] == mutableText[mutableText.endIndex.predecessor()] {
+      let range = Range<String.Index>(mutableText.startIndex.successor()..<mutableText.endIndex.predecessor())
+      return palindromeCheck(mutableText.substringWithRange(range))
+    }
+  }
+  
+  return false
+}

Palindromes/Palindromes.swift

@@ -0,0 +1,64 @@
+# Palindromes
+
+A palindrome is a word or phrase that is spelled the exact same when read forwards or backwards. 
+For this example puzzle, palindromes will not take case into account, meaning that uppercase and
+lowercase text will be treated the same. In addition, spaces will not be considered, allowing 
+for multi-word phrases to constitute palindromes too. 
+
+Algorithms that check for palindromes are a common programming interview question. 
+
+## Example 
+
+The word "radar" is spelled the exact same both forwards and backwards, and as a result is a palindrome. 
+The phrase "race car" is another common palindrome that is spelled the same forwards and backwards. 
+
+## Algorithm 
+
+To check for palindromes, the first and last characters of a String must be compared for equality. 
+When the first and last characters are the same, they are removed from the String, resulting in a 
+substring starting with the second character and ending at the second to last character. 
+
+In this implementation of a palindrome checker, recursion is used to check each substring of the 
+original String. 
+
+## The code 
+
+Here is a recursive implementation of this in Swift: 
+
+```swift
+func palindromeCheck (text: String?) -> Bool {
+  if let text = text {
+    let mutableText = text.stringByTrimmingCharactersInSet(.whitespaceCharacterSet()).lowercaseString
+    let length: Int = mutableText.characters.count
+    
+    guard length >= 1 {
+      return false
+    }
+
+    if length == 1 {
+      return true
+    } else if mutableText[mutableText.startIndex] == mutableText[mutableText.endIndex.predecessor()] {
+      let range = Range<String.Index>(mutableText.startIndex.successor()..<mutableText.endIndex.predecessor())
+      return palindromeCheck(mutableText.substringWithRange(range))
+    }
+  }
+
+  return false
+}
+```
+
+
+This code can be tested in a playground using the following: 
+
+```swift
+palindromeCheck("Race car")
+```
+
+Since the phrase "Race car" is a palindrome, this will return true. 
+
+## Additional Resources
+
+[Palindrome Wikipedia](https://en.wikipedia.org/wiki/Palindrome)
+
+
+*Written by [Stephen Rutstein](https://github.com/srutstein21)*

Palindromes/README.markdown

@@ -183,6 +183,7 @@ A lot of software developer interview questions consist of algorithmic puzzles.
 - [Two-Sum Problem](Two-Sum Problem/)
 - [Fizz Buzz](Fizz Buzz/)
 - [Monty Hall Problem](Monty Hall Problem/)
+- [Finding Palindromes](Palindromes/)
 
 ## Learn more!