In the early ’80s, when the city tried to rebuild the Wollman skating rink, it was supposed to take two years. Six years later, a real-estate developer — his name was Donald Trump — shamed the city into letting him take over, completing it in four months.

Now, the city and state have a new Wollman Rink: the Hudson River Park, under construction for 17 years. Time to get it done.

In one way, the Hudson River Park is a “yuge” success. Forty years ago, the West Side Highway from Battery Park to 59th Street was a wasteland littered with burned-out cars and a rusted roadway above. Today, it’s a 550-acre green space with dog parks, playgrounds and grass to read on. The country’s busiest bike lane runs alongside it.

That’s because previous governors and mayors thought ahead.

Gov. Cuomo’s father, Mario, set aside the land. In the mid-90s, then-Gov. George Pataki and Mayor Rudy Giuliani committed money to build it — $363 million so far, split between the city and state. And it’s paid off.

“The Hudson River Park is the most important physical transformation of Manhattan of the last 80 years,” says Adrian Benepe, who ran all of the rest of the city’s parks under Mayor Michael Bloomberg.

This is not an exaggeration. Nobody would want to live isolated, with nothing for their kids to do. But the park has made the Far West Side a real neighborhood.

The area has added 54 percent to its population over the past 15 years, including 4,300 little kids, even as the number of children elsewhere below 59th Street has fallen, says the Regional

Plan Association, which has done some research for the Friends of Hudson River Park, a nonprofit.

Its forthcoming study notes that the park has more than paid for itself, encouraging people (like me) to live and work near the waterfront, and encouraging developers to build for them.

The problem? Construction work went quickly from 1999 until 2012 or so — so that most of the park is usable (and nice). But now, the work is only 70 percent complete — and it’s stalled.

Walking and biking areas are still a patchwork. Some people have a patch of grass to suntan on, some don’t.

Gov. Cuomo and Mayor de Blasio aren’t as interested in finishing their predecessors’ legacies.

The Hudson park raises some money for day-to-day costs, from donations as well as commercial activity. But the commercial stuff always runs into activist opposition, adding to delays.

As city and state money slowed, the ability to finish it is hindered, says Scott Lawin, of the Friends of Hudson River Park and a local resident. “You might start to lose that value with a lot of unfinished spots, like a downtown with a lot of vacant storefronts,” says Robert Freudenberg of the RPA.

Unlike for Central Park, it’s hard to get private donors to pay for the remaining Hudson River stretches, about $150 million. “Central Park is surrounded on three sides by millionaires and billionaires,” Benepe said.

But one side of the Hudson River Park is, well, a river. “There are no wealthy and philanthropic striped bass,” says Benepe. The West Side, too, is less dense and has more middle-class residents who enjoy the park but can’t give millions.

“The Central Park Conservancy is a model Friends of Hudson Park aspires to,” says Lawin. But Central Park was complete, and had a century-old history, before people started donating.

The solution: Cuomo and de Blasio should commit to splitting the cost — $25 million each over three more years — to finish it. We’ve got surpluses for the moment, and there could hardly be a less controversial way to spend taxpayer money.

Taxpayers use the parks.

Central Park, Governors Island and the Hudson River Park are packed with too many people in the summer. Many Far West Siders won’t go near the High Line anymore (too trendy), so the Hudson River Park is really their neighborhood park.

Plus, it’s good to finish what you start. De Blasio wants to build a streetcar for Brooklyn waterfront neighborhoods similar to the Far West Side’s (the bike lane’s our streetcar).

How would he feel if his successor left it with tracks missing? We would have to call in . . . President Trump.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.