SAP智力題 Five people are smiling ★★★★★★

Five people, there is an integer on each one's head. Everyone can see other people's number but cannot see his own number, and each one can give other people a hint by choose either smiling or not. The number range is from 0 to N. And they are sure that at lease one can make a correct guess on his own number. What might be the greatest N? 

 

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