My first attempt was a brute force attempt. I didn't even try to come up with the (time) complexity of this, at least while I was on-site. // in Objective C everything is 0 based... but on my white board // my tic-tac-toe grid object was 1 based... // we might want to check if the game is valid even before // we waste time looking to see if there's a winning set of three - (BOOL) isValidGame: (Grid *) grid { int totX = 0; int totO = 0; for( int i = 1; i 1 ) return NO; return YES; } // this is the *brute force* solution - (NSString *) whoIsAWinner { int winnerX, winnerO; // initialize "winner" to zero... there can be at most only one winner winnerX = winnerO = 0; // first check for three X's or three Y's in a column for( int column = 1; column 1) || (winnerO > 1) { return NULL; } // tie - no winners if((winnerX == 0) && (winnerO == 0)) { return NULL; } if(winnerX == 0) && (winnerO == 1) { return @"O"; } if(winnerX == 1) && (winnerO == 0) { return @"X"; } return NULL; } Another approach I spoke about (and would have worked up had I had time within the one hour window) would have been to have separate all possible lines of three into different groups. E.G. [1,1][1,2][1,3]; [2,1][2,2][2,3];…;…;[1,1],[2,2],[3,3];[3,1][2,2][1,3] And then look for three matching symbols within each set of triplets.