Three days ago, Jalyn Dunlap could hardly walk. Today, he’s walking on air.

“I didn’t even know if he could play,” Westbury coach Dave Graff said, referencing what he said was a groin injury for his top sub.

“It still hurts, but the trainers did a great job to get me ready. I couldn’t sit out this one,” said Dunlap, a 5-11 junior guard.

After a missed free throw, Dunlap grabbed the offensive rebound in traffic and made a bank shot in the lane with 10 seconds left to give Westbury a thrilling 61-60 victory over Half Hollow Hills East yesterday in a memorable Long Island Class AA championship game played before a roaring crowd of more than 2,000 fans at Stony Brook University’s Island FCU Arena.

Westbury’s Jonathan Dean, who had scored six of his 16 points in the fourth, missed a one-and-one with 13.2 seconds left, but Dunlap, as he had all game, came to the rescue.

He saw the ball bounce high to the left, leaped to corral the rebound between two defenders, turned and banked it home for a 61-60 lead. The Thunderbirds raced upcourt with enough time to get off a shot, but a short jumper in the lane rimmed out, triggering a wild, ear-splitting hug-and-holler scene among Westbury players and fans on the court.

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Westbury (20-3) advanced to the state semifinals Saturday in Binghamton for the first time as a Class AA school. Hills East, which won the school’s first Suffolk boys basketball title, finished 21-5 and received outstanding play from its junior backcourt. Julien Crittendon hit five three-pointers and scored 15 of his 27 points in the second half. Speedy point guard Savion Lewis added 12 points.

Dunlap netted 17 of his 24 points in the second half. “It hasn’t hit me yet. This is unreal,” he said.

His first real contribution came in the second quarter when he entered with Westbury trailing 28-13. He converted two putbacks, turning one into a three-point play, as the Green Dragons closed out the half on a 13-2 run to cut it to 30-26. He also beat the buzzer with a pull-up jumper to give Westbury a 45-43 edge after three.

Hills East seemed to have regained control when Kendall Nero-Clark scored back-to-back baskets on slick feeds from Crittendon and Lewis, then pulled up for a jumper with 2:47 left for a 56-51 lead. After Kenny Mathurin scored inside to make it 58-53 with 2:03 left, Graff called a timeout.

“We set up a play, scored and put on the press,” he said.

Dunlap hit a jumper after the timeout and followed that with a twisting, up-and-under move that made it 58-57. Crittendon hit two free throws for a three-point T-birds lead, but Isaiah Bien-Aise’s putback with 16 seconds left cut the margin to one. Hills East and Westbury both missed one-and-ones, but Dunlap turned the Green Dragons’ miss into a golden moment. “Jalyn was sensational,” Graff said.

Pretty spry, too, aching groin or not.