okay, take a look at the tree in the pic above. if you remove node 7, what is the complexity of merging the tree? Put the node 7 as a child to node 5, now how much time will it take?
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Used the same approach in the contest… Got a TLE! The NOP idea did not strike! Nice one 
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okay, I think I get it. Thanks
My O(N^2) algo failed!! Someone is lying!
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You can try submit my code and check
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even I used the same concept but got WA, missed some case somewhere I think. Onto it now. Nice exp btw