Purdue is national championship-good this season, looking every bit like an eventual 1-seed when Selection Sunday rolls around.

But how did this happen? The Boilermakers were supposed to be good, but not this good. Losing Caleb Swanigan — the most dominant big man in college basketball last year — was expected to sting a little more. Swanigan was the Big Ten Player of the Year, a first-team All-American — an offense unto himself. Purdue isn’t some one-and-done factory that should be so easily able to replace elite talents.

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The Boilermakers are significantly better this year, though. Outside of Swanigan, they brought pretty much everyone back and added 7-3 freshman Matt Haarms — a shot-swatting, fist-pumping delight via Amsterdam that is the perfect change-of-pace to Isaac Haas.

The biggest reason for Purdue’s success? Balance. The Boilermakers rank third in offense and seventh in defense, the only team in America with a top-seven unit on both ends of the floor. Dig a little deeper, and Purdue’s downright competence is incredibly rare. Michigan State also has a top-10 offense and defense. Besides that duo, no team has a top-20 unit on both ends. Offense without defense doesn’t win championships; defense without offense doesn’t win championships. It takes both.

Let’s break down why Purdue has been so good in each facet of the game.

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*Offense

Purdue is making a ridiculous 44 percent of its 3s — tops in the nation — and has four elite shooters surrounding the 7-2, 290-pound Haas. Every starter besides Haas is making 40 percent of his 3s; Carsen Edwards, smack-dab at 40, is the worst of the bunch.

But Edwards is an elite shooter, the type of guy who would easily be most teams’ top sniper. Simply put: when the second-worst shooter in your starting lineup is awesome at it, your offense is going to be pretty great.

Vince Edwards (45 percent) Dakota Mathias (46) and P.J. Thompson (51) round out the starting five, and they see open looks often. Haas lives a charmed basketball life, by the way. He’s limited — which is why someone who’s putting up 27 and 10 per 40 minutes only plays 22 minutes a night. Haas fouls a lot and struggles to defend the pick-and-roll, and at his size, gets fatigued quickly.

But he’s one of the few true post-up mismatches that remains in college hoops. Guys like Haas are a dying breed. There’s a reason why the post-up is far less common than it used to be — it’s generally an inefficient shot, and we’ve gotten smarter. In order for a post-up to really be worth it, the offensive player must have a clear advantage over the defensive player. Pit a 6-10, 240-pounder against another 6-10, 240-pounder, and you’re really just spinning your wheels.

But Haas has a physical advantage over like, everyone, and possesses the touch and footwork to make that matter. Moritz Wagner is no dwarf — he’s 6-11, 245 pounds and can brutalize a post mismatch in his own right.

Haas ate him alive in Purdue’s 92-88 win over Michigan, scoring 24 points in 20 minutes. If the goal of modern offense is to generate 3s and layups, Haas was able to establish such deep position that his post-ups essentially were layups:

Purdue outlasts Michigan, 92-88

Michigan went with the ‘we’re not going to let the shooters beat us’ strategy. Fine. Then Haas is going to score more than a point per minute. The Boilermakers present foes with a painstaking dilemma — double team Haas, and run the risk of leaving a 40 percent (or better) 3-point shooter wide open. Don’t double Haas, and you get eviscerated on the inside like the Wolverines did (for what it’s worth, that’s probably the right strategy from a math perspective. It also had to be maddening for John Beilein to watch Haas lay the ball in possession after possession).

This is why fit matters so much. Haas wouldn’t be nearly as effective if Matt Painter played Haarms next to him, but Painter doesn’t. Imagine if Deandre Ayton — also a dominant big — played in an offensive scheme like Haas. Instead, Ayton has to share the floor with Dusan Ristic — a nice player, but someone who operates in the same territory as Ayton. Haas has no such issue. Purdue’s personnel fits together perfectly.

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Most of this was true with Swanigan on board — and with another year of experience, the shooters have become more accurate. But Carsen Edwards is the main reason why the Boilermaker offense is more dynamic than ever. He’s the one player they have who can consistently break guys down off the dribble.

2-pointer by Carsen Edwards

The problem last year? He was an inefficient gunner, still learning the nuances of the college game. Edwards is nearly shooting a full 10 percent better than he did as a freshman. Thompson — who ranks fifth in the country in offensive rating — is an awesome second guard. But you’re probably not winning a title with him as your lead ball-handler. Last year, the numbers didn’t reflect Edwards’ talent. They are in 2017-18; if opponents somehow find an answer for the ‘Haas surrounded by four shooters’ offense, Edwards is the kind of shot-creator that can bail Purdue out of a sticky situation.

*Defense

Purdue’s offense didn’t improve because Swanigan left. Several Boilermakers needed to develop in order for that to happen. To their credit, they did.

But Swanigan was a below-average to mediocre defender. Replacing so many of his minutes with Haarms, an elite rim-protector who’s nimble in defending ball screens, is the main reason why the defense has jumped from 23rd to seventh.

Haarms is averaging an astounding six blocks per 40 minutes. When Haas is on the floor, opponents can run him through ball screens all day and generally wind up with a good shot. Haas is a solid vertical rim-protector, but struggles to move laterally. The same was true of Swanigan.

Haarms, at 7-3, can stick with guards shockingly well for a guy his size:

Purdue has two ace defenders in Haarms and Mathias. Vince Edwards can be a standout defender when he’s locked into his assignment — he’ll space out at times — but is still a net plus. Thompson is always in the right place, and Carsen Edwards won’t hurt you. The Boilermaker defense has no obvious weak link when Haarms is on the floor.

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Purdue’s biggest issue? When Haas is in the game, they’re excellent on offense and merely ‘good’ on defense. With Haarms on the floor, that’s flipped. We mentioned Haarms’ exquisite defensive splits, but Purdue scores 111.2 points per 100 possessions when he’s in the game — a far cry from Haas’ 125.1 mark. The overall numbers say Purdue is elite on both sides of the floor. But it doesn’t have a specific lineup that can say that; the Boilermakers are really good for all 40 minutes because one of Haas or Haarms is on the floor at all times. Their crunch-time five depends on game flow.

But that’s a minor problem in the grand scheme of things. We can argue whether or not Purdue is the best team in the country, but it would be tough to dispute that it’s the most well-rounded.

We’ll see what the Boilermakers can do in March — a month that hasn’t been too kind to them lately. That was once true of Villanova, and we saw how 2016 turned out.

Purdue has all of the ingrdients to win at the highest level.