DELAND, Fla. — Don’t be fooled by the smile, the laid-back approach or the long curly brown hair.

Three words are all you need to know about Jacob deGrom as he prepares for an encore to his stunning Rookie of the Year season.

“My dad always had a saying — it’s from the movie ‘Remember the Titans.’ There’s a part where they say, ‘Leave no doubt,’” deGrom told the Post this week at his former college, Stetson University, where coach Pete Dunn had the good sense to move deGrom from shortstop to the pitching mound his junior season.

“My dad used to say that before games,” deGrom recalled, “so I guess I would go out there and try to do that: Leave no doubt.”

Those are the words the Mets’ right-hander lives by every time he takes the mound. DeGrom is as focused and as determined as Matt Harvey, he just does it a much different way.

“I’ve always been competitive,” deGrom said. “I don’t like losing. I feel that when someone steps into the box it is me versus. him. I want to win that battle.”

DeGrom’s dad Tony, 60, is a gifted athlete as well and still plays shortstop in a softball league.

“I went to his game the other night and he actually hit a home run over the fence,” deGrom said with a smile. “I still throw with my dad every day.”

DeGrom, 26, lives in nearby DeLeon Springs, about 10 minutes from his parents’ home, and is trying to take in what all his rookie success means.

“I still kind of have to pinch myself to make sure it’s real,” deGrom admitted. “When I think about it, it’s kind of crazy looking back at this past year and all the stuff that happened and then winning the Rookie of the Year Award. It’s taken some getting used to, getting recognized and stuff.

“I try to keep things simple, try not to let any of that change me.”

It will be impossible for 2015 to match up to 2014 for deGrom, not only because he came out of nowhere to post a 9-6 record with a 2.69 ERA while striking out 144 batters over 140 ¹/₃ innings.

Most importantly, he was married to Stacey Harris from nearby Lake Helen in early November. Then came the honeymoon in Maui.

The couple first met at a bull-riding event in Ocala.

“We met through mutual friends,” deGrom noted. “She’s kind of like me, easy going.”

On the field, deGrom knows one sure-fire way 2015 could top 2014, and that is to play in October.

Just like Mets teammates David Wright and Lucas Duda said earlier in the week in Port St. Lucie, deGrom fully expects the Mets to be in the postseason.

That’s what good, young arms can do for an organization — especially with the return of Harvey from Tommy John surgery, something deGrom went through in 2010.

“I think it’s going to be a great year,” deGrom said. “With David healthy, Harvey coming back, [Zack] Wheeler, and then we signed [Michael] Cuddyer, I think this year is going to be a lot of fun. We should have a really good team. I think for sure we should be playing in October.

“Everybody plays to go to the World Series. Hopefully it all comes together this year.”

DeGrom does not want to mess with success in any way. Hair today. Gone tomorrow. He has no plans to get a haircut. He will get it “cleaned up a little probably just before spring training.’’ His sister Jessica does the honors.

DeGrom never imagined his hair would become such a New York thing.

“There’s been a few times that I’ve been pretty aggravated with it, getting in my face, and I want to cut it off but I figure I better not,’’ he said. “I don’t know, maybe the ball hides behind it a little bit, so I’m going to keep it.”

His opportunity came last season when Dillon Gee got hurt.

“I kind of thought to myself that some guys only get one chance so make the most of it,” deGrom said.

Leave no doubt.

“Man, was I nervous that first game,” deGrom said. “It was against the Yankees and [Derek] Jeter stepped into the box and I said, ‘OK, this is real.’

“I played shortstop, that was the guy I followed. He was probably the best shortstop to play the game, so when he stepped in there, that was a really cool night.”

DeGrom switched from shortstop to closer his final season at Stetson, 2010.

Noted Dunn, “We sat down as a staff and said, ‘Our best arm is out at shortstop.”

Soon, deGrom became the Friday night starter, the ace.

Corey Kluber also pitched at Stetson. He was drafted by the Indians in 2007 and won the AL Cy Young Award in 2014.

“Both are very laid back, but they are also both very competitive inside, a burning desire to compete,’’ Dunn said of his pair of aces.

“I was kidding Jacob the other day, I said, ‘Man, don’t change, just keep that DeLeon Springs redneck personality and you’ll be fine,’” Dunn said with a laugh. “He doesn’t get caught up in himself. He hasn’t changed a bit. He is driving a lot nicer truck now, though. But he’s earned that.”

That’s a Ford F-250 with monster wheels.

At Stetson, deGrom matched up against Florida Gulf Coast’s Chris Sale, now a star hurler for the White Sox, in the conference tournament in 2010. He hit a home run off him.

“That was my only home run in college,’’ deGrom said with a smile. “My second college start was against [Sale], and that is when I started getting noticed.”

Scouts came to see Sale and walked away with another pitcher to follow. The Mets took the junior in the ninth round of that 2010 draft. In October of that year it was determined he needed Tommy John surgery in New York.

DeGrom called his mother Tammy and said, “Don’t worry, I’ll be home tomorrow. I’ll be fine. I’m glad I got it out of the way.”

He got a buzz cut at the time of the surgery. He hasn’t gotten his hair cut that short again. Opposing fans let him know it. “Get your hair cut,” they often yell.

“It’s like, ‘All right, that’s a new one,’” deGrom said.

Now, deGrom is prepped to make the most of his second season. His elevated fastball is a difference-maker. Same goes for the Giants World Series hero Madison Bumgarner. Here it comes.

“I’m going to go out there and compete,” deGrom promised.

Just like he did his rookie season.

Leave no doubt.