how can i prove that this grammar is ambiguous?

S -> bA|aB
A -> a|aS|bAA
B ->开发者_如何学编程 b|bS|aBB

Any easy method other than trying to find a string that would generate two parse trees ?

Can someone please give me a string that can prove this.

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There is a string though: bbaaba

S -> bA -> bbAA -> bbaA -> bbaaS -> bbaabA -> bbaaba
S -> bA -> bbAA -> bbaSA -> bbaaBA -> bbaabA -> bbaaba

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There is no easy method for proving a context-free grammar ambiguous -- in fact, the question is undecidable, by reduction to the Post correspondence problem.