During his final public event here, Obama basked in the admiration of hundreds of young leaders who participated in a town hall-style event and prefaced some of their questions to him with praise about his leadership and his "inspiring speeches."
Obama used a question about preserving a Vietnamese cave from development to pivot to climate change, one of his top issues as president. He said Vietnam will be one of the countries most affected by the trend of warming temperatures and rising seas.
"One of the great things about your generation is that you're already much more conscious about the environment than my generation was or previous generations were," said Obama.
He told a previous questioner that he "fooled around a lot" and wasn't serious about school when he was young.
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Obama also promoted a 12-nation, trans-Pacific trade pact that includes Vietnam but is stalled in the US Congress and opposed by the three leading US presidential candidates.
But he avoided wading too deeply into politics when asked where he sees himself and the world in five years, around the end of his successor's first term. Obama has said on previous occasions that world leaders ask him all the time about the unpredictable election.
"Sometimes, our politics doesn't express all the goodness of the people, but usually, eventually, the voters make good decisions and democracy works," Obama said. "So I'm optimistic that we'll get through this period."