If you see a girl you really like in public but she's walking away how do you approach her before its too late?

I've made the mistake of seeing a girl I like but don't say anything and then she's gone. I don't want to run , raise my voice in desperation, and say "wait a second, can I talk to you? "

I feel thats startling and she has no clue who I am. Still its better to try than not but I don't know how to go about it.
Go after her and say something!
Let it go and move on.
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If you see a girl you really like in public but she's walking away how do you approach her before its too late?
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