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"How many variables do you need to keep track?",
"Two variables is all you need. Try with <code>x</code> and <code>y</code>.",
"Beware of empty rows. It could be the first few rows.",
"To write correct code, think about the <a href=\"https://en.wikipedia.org/wiki/Invariant_(computer_science)\" target=\"_blank\">invariant</a> to maintain. What is it?",
"The invariant is <code>x</code> and <code>y</code> must always point to a valid point in the 2d vector. Should you maintain your invariant <i>ahead of time</i> or <i>right when you need it</i>?",
"Not sure? Think about how you would implement <code>hasNext()</code>. Which is more complex?",
"Common logic in two different places should be refactored into a common method."
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