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Comment by verroq | Hacker News Reader

Here is how I did it

    start: left
           skipNext
           goto start
           goto double

    double: left
            left
            goto double

The idea comes from the "find a loop in a singlely linked list in O(1) space" problem.

One pointer moves twice as fast the as the other one.

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You don't need the 'goto double' line, execution is anyway going skip to 'double: left' making the line redundant

Also say each step takes one unit of time, so one 'start' loop requires 3 units to go one unit of distance, whereas 'double' takes 3 units to go 2 units of distance, even if you use one left statement in the 'double' loop, robot will still move faster than the other as it will require only two units of time to go one unit of distance.

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