Cassie was out and about in NYC, flashing her smile and soaking up the good vibes ... while her ex Diddy is stuck in a whole different reality -- lonely and behind bars in a notoriously grimy Brooklyn prison.

The singer looked at ease as she headed to the trendy hotspot "Left Bank" for an al fresco dinner Wednesday ... where the fun continued as she appeared to excitedly show her pals something on her phone at the dinner table.

While we can't say for sure, it’s a safe bet that Diddy came up in conversation at some point, especially since he was sensationally denied bail on the very same day.

Whether Diddy was discussed or not, Cassie finally seems to be basking in her path to justice, especially after the 2016 hotel surveillance footage of the disgraced mogul hitting her surfaced months earlier.

Though Cassie wasn’t directly named, the hotel footage was cited by Assistant U.S. Attorney Emily Johnson in court as she addressed Diddy’s “Freak Off” parties ... alleging he tried to force a woman into having sex with an escort back in 2016.

The prosecutor argued that the alleged incident with Cassie shows Diddy can't be trusted and suggested the half-dozen sex workers tied to the case are just the "tip of the iceberg."