an algorithm question from a recent interview: given an array (for example, [ 1, 0, 2, 0, 0, 3, 4 ]), implement a method 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

SortAllZerosToEndOfArrayAndReturnCountOfNonZeroElements.m

/*
Given an array (for example, [ 1, 0, 2, 0, 0, 3, 4 ]), implement a method 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
*/

@interface FB : NSObject

@end

@implementation FB

- (NSInteger) countAndRearrangeNonZeroElementsFromThisArray: (NSMutableArray *) originalArray
{
    NSInteger countOfZeros = 0;
    NSInteger indexOfLastNonZeroItem = [originalArray count] - 1;
    
    for(NSInteger index = 0; index < [originalArray count]; index++)
    {
        NSNumber * numberFromArray = [originalArray objectAtIndex:index];
        
        if([numberFromArray intValue] == 0)
        {
            countOfZeros++;
            [originalArray exchangeObjectAtIndex:indexOfLastNonZeroItem withObjectAtIndex:index];
            indexOfLastNonZeroItem--;
        }
        
        if(index > indexOfLastNonZeroItem)
            break;
    }
    return(countOfZeros);
}

@end

int main (int argc, const char * argv[])
{
    @autoreleasepool {
        
        FB * fbObj = [[FB alloc] init];
        
        if(fbObj)
        {
            NSMutableArray *mutableArray = [@[@1, @0, @2, @0, @0, @3, @4 ] mutableCopy];
            NSInteger numberOfZeros = [fbObj countAndRearrangeNonZeroElementsFromThisArray:mutableArray];
            
            NSLog(@"numberOfZeros = %ld %@", numberOfZeros, mutableArray);
        }
    }
}