Shouldn't we need to use a while loop to see if the window has become invalid rather than an if statement? Consider the situation in which arr[windowStart] = 1 when we enter the if statement. In that case, we'll decrement maxOnesCount and also increment windowStart. This means that when we leave the if statement, we will still have an invalid window (i.e. windowEnd - windowStart + 1 - maxOnesCount will still be greater than k).