Ios engineer Interview Questions
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Ios Engineer interview questions shared by candidates
What are the App states. Explain them?
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Active: app is foreground Background: app is not foreground but still executing codes Inactive: app is suspended Less
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Not Running - app has not been launched or was running but terminated by system. Inactive - app is running in foreground but not receiving events. Active - app is running in foreground and receiving events. Background - app is in the background and executing code. Suspended- the app is in the background but is not executing code. Less
What is a volatile variable, and why would you use it?
2 Answers↳
Consider a single threaded single processor environment. Do you still need a volatile? Less
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Volatile keyword is used during multi-threading when you want the threads to edit this variable value. It ensures that the value of the variable is not cached and is stored directly in the main memory. hence, enabling concurrent usage of the variable by different threads. Less
What is iphone reference library?
1 Answers↳
iPhone reference library is a set of reference documents for iPhone OS.
Phone interview question # 1: Given an array (for example, [ 1, 0, 2, 0, 0, 3, 4 ]), implement methods that 1. returns the number of non-zero elements (4) 2. moves the non-zero elements to the beginning of the array (the rest of the elements don't matter) -> both [ 1, 2, 3, 4, 0, 0, 0] and [ 4, 1, 3, 2, X, Y, Z ] are valid
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let givenArray = [1,0,2,0,0,3,4] var arrayWithNonZeroElements = givenArray.filter({ $0 != 0 }) let arrayWithZeroElements = givenArray.filter({ $0 == 0 }) print(arrayWithNonZeroElements.count) //"4\n" arrayWithNonZeroElements.appendContentsOf(arrayWithZeroElements) print(arrayWithNonZeroElementst) // [1,2,3,4,0,0,0] Less
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The answer from OP is good, but there is a case where it doesn't work : if the last value of the array is @0 and the array contains another @0 (Ex: [@1, @0, @2, @0], then you end up swapping two @0 and not solving fully the problem. There is a little optimization possible on the ending condition of the for loop : ending when we reach exchangeObjectAtIndex:indexOfLastNonZeroItem instead. Here is my full solution : - (NSInteger) countAndRearrangeNonZeroElementsFromThisArray: (NSMutableArray*) originalArray { NSInteger countOfZeros = 0; NSInteger indexOfLastNonZeroItem = [originalArray count] - 1; while ([originalArray[indexOfLastNonZeroItem] isEqualToNumber: @0]) { indexOfLastNonZeroItem--; } for(NSInteger index = 0; index < indexOfLastNonZeroItem; index++) { NSNumber * numberFromArray = [originalArray objectAtIndex:index]; if([numberFromArray isEqualToNumber:@0]){ countOfZeros++; [originalArray exchangeObjectAtIndex:index withObjectAtIndex:indexOfLastNonZeroItem]; indexOfLastNonZeroItem--; } } return countOfZeros; } Less
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modified version of the above answer, that works with inout arrays func countZerosAndMoveToBegning(array: inout [String]) -> UInt { let nonZeros = array.filter { $0 != "0" } let zeros = array.count - nonZeros.count let arrayZeros = Array(repeating: "0", count: zeros) array.removeAll() array.append(contentsOf: arrayZeros) array.append(contentsOf: nonZeros) return UInt(zeros) } Less
Given an array, without using extra space, move all zeros to the end and no-zeros to the beginning. The function should return the number of non-zeros.
14 Answers↳
-(int)sortZerosAndReturnCountOfThem:(NSMutableArray *)arr { NSLog(@"before: %@", arr); int cursorFromStart = 0; int cursorFromEnd = (int)arr.count - 1; while (cursorFromStart < cursorFromEnd) { while (arr[cursorFromStart].intValue != 0) { cursorFromStart++; } while (arr[cursorFromEnd].intValue == 0) { cursorFromEnd--; } if (cursorFromStart < cursorFromEnd) { arr[cursorFromStart] = arr[cursorFromEnd]; arr[cursorFromEnd] = @(0); } } NSLog(@"after: %@", arr); int counter = 0; for (NSNumber *num in arr) { if (num.intValue == 0) { counter++; } } NSLog(@"counter: %d", counter); return counter; } Less
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func moveZeros(input: inout [Int]) -> Int { //reverse sort input = input.sorted { $0 > $1 } let countNonZero = input.filter { $0 != 0 }.count return countNonZero } Less
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No need to sort. Can do in O(n) time. Swift: func moveAllZerosToFront(list: inout [Int]) -> Int { var numOfZeros = 0 for i in 0.. Less
Verify that a binary search tree is indeed a binary search tree.
12 Answers↳
The idea is if we traverse binary search tree "in-order", output of number is in ascending orders. So here it goes. Not checked for syntax errors, but should work. BOOL Inorder(NODE *node) { static NODE *prev = NULL; // Do need to initialize this as statics are zeroed memory. if (node == NULL) { return true; } if (false == Inorder(node->left)) { return false; } if (prev && node->num num) { return false; } prev = node; return Inorder(node->right); } Less
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A simpler approach is to do an inorder traversal keeping track of last seen element. If it is always increasing then we can prove that tree is BST. Less
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class TreeNode { var value: Int? var leftChild: TreeNode? var rightChild: TreeNode? } func isBST(node: TreeNode, min: Int, max: Int) -> Bool { print("value: " + String(node.value!)) print("min: " + String(min)) print("max: " + String(max)) if node.value! max { return false } if let leftChild = node.leftChild { if isBST(leftChild, min: min, max: node.value!) == false { return false } } if let rightChild = node.rightChild { if isBST(rightChild, min: node.value!, max: max) == false { return false } } return true } // Creating BST let minValue = Int.min let maxValue = Int.max let node2 = TreeNode() node2.value = 2 node2.leftChild = nil node2.rightChild = nil let node7 = TreeNode() node7.value = 7 node7.leftChild = nil node7.rightChild = nil let node11 = TreeNode() node11.value = 11 node11.leftChild = nil node11.rightChild = nil let node15 = TreeNode() node15.value = 15 node15.leftChild = nil node15.rightChild = nil let node5 = TreeNode() node5.value = 5 node5.leftChild = node2 node5.rightChild = node7 let node13 = TreeNode() node13.value = 13 node13.leftChild = node11 node13.rightChild = node15 let node10 = TreeNode() node10.value = 10 node10.leftChild = node5 node10.rightChild = node13 if isBST(node10, min: minValue, max: maxValue) == true { print("Tree is BST.") }else { print("Tree is not BST.") } Less
- Given an array, remove the duplicates and return a unique array keeping the first occurrence of the duplicates and the order. [@2, @1, @3, @1, @2] --> [@2, @1, @3] - how would you implement call for canceling queued blocks with dispatch_after?
12 Answers↳
There is a built in class that already does this. Just use that. NSArray *array; // input array NSArray *result = [[NSOrderedSet orderedSetWithArray:array] array]; Less
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the question answered above are definitely not optimized but easy built in function, I doubt thats what the interviewer want, if it's sorted array, it will be easy, we simply keep track of the current number, remove duplicate and move forward, but if it's not sorted, then I think a NSMutableDictionary would be efficient to perform a one time scan from begin to the end, so it's O(n) time and O(n) space. Less
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Is it alright if we use NSOrderedSet? Or do they expect you to solve it without using the built-in components. Because with NSOrderedSet, it can be solved in 1 line. Less
Given a list of number 8 4 3 0 6 4 0, move all 0's to start of the list. use minimum sorting rounds
10 Answers↳
swift solution using bucket sort func sortZeroesAtEnd(in array: [Int]) -> [Int] { //pseudo bucket sort var zeroBucket: [Int] = [] var nonZeroBucket: [Int] = [] for n in array { if n == 0 { zeroBucket.append(n) } else { nonZeroBucket.append(n) } } return nonZeroBucket + zeroBucket } Less
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var array = [8, 4, 3, 0, 6, 4, 0] var offset = 0 for index in stride(from: array.count - 1, through: 0, by: -1) { if array[index] == 0 { offset += 1 } else { array[(index + offset)] = array[index] if index < offset { array[index] = 0 } } } // result: [0, 0, 8, 4, 3, 6, 4] Less
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func moveZerosToFront(array: [Int]) -> [Int] { var resultArray = [Int]() for num in array { num == 0 ? resultArray.insert(0, at: 0) : resultArray.append(num) } return resultArray } Less
How to detect a common superview.
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- (nullable UIView *)commonSuperview:(nullable UIView *)otherView { if(!otherView) return nil; NSMutableArray* superViewsArray = [NSMutableArray array]; UIView* superVeiw = self; while (superVeiw) { [superViewsArray addObject:superVeiw]; superVeiw = superVeiw.superview; } superVeiw = otherView; while (superVeiw) { if([superViewsArray containsObject:superVeiw]){ break; } superVeiw = superVeiw.superview; } return superVeiw; } Less
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extension UIView { func sameSuperview(other: UIView) -> UIView? { guard self.superview != nil || other.superview != nil else { return nil } var superViews: [UIView] = [] //Should be a set var selfSuperView = self.superview while selfSuperView != nil { superViews.append(selfSuperView!) selfSuperView = selfSuperView!.superview } selfSuperView = other.superview while selfSuperView != nil { if superViews.contains(selfSuperView!) { return selfSuperView } selfSuperView = selfSuperView!.superview } return nil } } Less
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- (UIView *)commonSuperView:(UIView *)oneView anotherView:(UIView *)anotherView { if (!oneView || !anotherView) { return nil; } NSMutableArray *viewArray = @[oneView].mutableCopy; while (oneView.superview) { oneView = oneView.superview; [viewArray addObject:oneView]; } if ([viewArray containsObject:anotherView]) { return anotherView; } while (anotherView.superview) { anotherView = anotherView.superview; if ([viewArray containsObject:anotherView]) { return anotherView; } } return nil; } Less
given an array of words, how to eliminate the duplicates? You have a simple tree structure Ⓐ and its clone ⓐ. Each node in the tree has a pointer to it's parent as well as an array of its children. Given an original tree's node Ⓑ and cloned tree ⓐ, implement a method that returns ⓑ (the clone of Ⓑ). (Imagine finding the matching UIButton/UISlider/UIView in a separate cloned view controller.) Original Ⓐ ┏━┻━━┓ ◯ ◯ ┏┻┓ ┏━╋━┓ ◯ ◯ ◯ ◯ ◯ ┏┻┓ ┃ ◯ Ⓑ ◯ Clone ⓐ ┏━┻━━┓ ◯ ◯ ┏┻┓ ┏━╋━┓ ◯ ◯ ◯ ◯ ◯ ┏┻┓ ┃ ◯ ⓑ ◯
8 Answers↳
Since nobody answered regarding the UiView tree question, posting my solution that I finished only after the interview: -(void)originalInput { UIView *B = [UIView someViewWithParentAndChildren]; //known subview UIView *a = [UIView someViewWithParentAndChildren]; //root of clone view tree NSMutableArray *indexPaths = [NSMutableArray new]; [self recordIndexPaths:indexPaths fromView:B]; //creating index array UIView *cloneViewB = [self findViewByIndexes:indexPaths forIndex:0 forView:a]; //recursing through the clone view hierarchy NSLog(@"%@", cloneViewB); } -(void)recordIndexPaths:(NSMutableArray *)indexPaths fromView:(UIView *)view { if (view.parent == nil){ return; }else { UIView *parent = view.parent; int i = [parent.children indexOfObject:view]; [indexPaths insertObject:i atIndex:0]; //i is not and object, but primitive, should be incapsulated in NSNumber in real life!! [self recordIndexPaths:indexPaths fromView:parent]; } } -(UIView *)findViewByIndexes:(NSArray *)indexPaths forIndex:(int)i forView:(UIView *)currentView { if (!currentView.children.count) return currentView; int j = indexPaths[i]; //j is not and object, but primitive, should be incapsulated in NSNumber in real life!! UIView *child = currentView.children[j]; return [self findViewByIndexes:indexPaths forIndex:i+1 forView:child]; } Less
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// We just need to find the path to the main top superview on the original view, saving the index of each view based on its superview and then reverse the path and search on the cloned view. func findView(_ view: UIView, in clonedView: UIView) { var path = [Int]() var view: UIView? = view while view != nil { let tmp = view! view = view!.superview if let idx = view?.subviews.index(of: tmp) { path.append(idx) } } view = clonedView for idx in path.reversed() { view = view?.subviews[idx] } print("Cloned view is: \(view!.tag)") } Less
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func removeDups(words: [String], caseSensitive: Bool = false) -> [String] { var hashtable = Dictionary() var words2 = [String]() for word in words { let hash = (caseSensitive ? word : word.uppercased()).hashValue hashtable[hash] = (hashtable[hash] ?? 0) + 1 if hashtable[hash] == 1 { words2.append(word) } } return words2 } Less