According to the World Federation of Exchanges, there were 45,358 publically traded companies worldwide at the end of 2009.                                                                  

Asked years ago how he finds gems when so many companies exist, Warren Buffett said he "starts with the A's."

Impressive, but impractical for most of us. We need a faster, more efficient way to find good investment ideas.

One smart approach: Copy the masters. Watch what they do. Get your ideas from them, then dig a little deeper.

That's what we do several times a year, when the world's greatest investors are required to disclose what they bought and sold in the previous quarter. Here's what six of them have been up to recently.

John Paulson 
Virtually unknown a few years ago, Paulson is now one of the world's most successful investors. He made a fortune riding the housing bubble down, and then the economic recovery back up. His personal payday came out to about $100 per second over the past four years.                                            

Paulson's hedge fund opened a new 25-million-share stake in Hewlett-Packard (NYSE: HPQ) last quarter, worth about $900 million at today's prices. Shares suffered a bit of crash yesterday, but now trade at about 6.5 times next year's earnings, and less than five times cash flow. Paulson also added to his already massive bet on gold, more than doubling his stake in Barrick Gold. Earlier this month, he made goldbugs' hearts go pitter-pat by predicting that the yellow metal is on its way to $4,000 an ounce.

Warren Buffett 
One has to be careful evaluating the fluctuations in Berkshire Hathaway's (NYSE: BRK-B) portfolio, since transactions often come from Buffett's lieutenants.

In the past, trades made by GEICO investment manager Lou Simpson were often misperceived as Buffett's work. Simpson is recently retired, but possible Buffett successor Todd Combs is now managing several billion dollars of Berkshire's money.                                                                                             

Combs is almost certainly behind Berkshire's new position in MasterCard. He specialized in financial stocks at his previous hedge fund, and the new position is fairly small, worth about $60 million. Buffett hinted in the past that any new Berkshire positions worth less than $1 billion were probably initiated by someone other than him.

David Tepper 
Tepper made a fortune betting on bank stocks during the 2009 meltdown. He's now backing off those bets. Last quarter, Tepper's hedge fund pared down stakes in Bank of America (NYSE: BAC), Citigroup (NYSE: C), and Wells Fargo (NYSE: WFC).

His latest buys: airlines. Tepper held small stakes in Delta Airlines (NYSE: DAL) and US Airways before, but cranked up the heat last quarter, more than doubling his bet on Delta and purchasing more than 10 million shares of US Airways.

You know the quip: What's the easiest way to become a millionaire? Start with a billion dollars and buy an airline. What's Tepper see in airlines? Who knows. They have regained pricing power after slashing capacity during the recession. Another possibility: betting on airlines is really a bet on falling oil prices.

George Soros 
It's been more than a year since George Soros called gold the "ultimate bubble," following up by noting that, "As a participant, when I see a bubble, I rush out and buy." And buy he did.

But his party's over. Soros's hedge fund sold almost all of its stake in the SPDR Gold Trust, and drastically cut its position in Novagold Resources. This is why Soros can buy bubbles. He sells before everyone else.

On the buy side: Soros's fund purchased over 1 million shares of Comcast during the quarter.

Bill Ackman 
An activist investor with a penchant for being either brilliantly right or catastrophically wrong, Ackman appears to have spent most of last quarter selling. Stakes in General Motors, ADP and Target were all liquidated.

David Einhorn 
Einhorn was busy adding new positions during the quarter, with new stakes in Best Buy, CVS Caremark, General Motors, and Yahoo! (Nasdaq: YHOO).

Einhorn laid out the bullish case for a couple of these new buys in a recent letter to his investors. On Best Buy:

Over the years we have seen many retailers given up for dead after a weak holiday result, only to recover with a change in fashion or product cycle. Bears believe that the Internet puts BBY on a path to Blockbuster-video obsolescence. We think that view overstates the risk as there is value in store help, merchandising, service and being able to walk out of the store with your purchase.

And Yahoo!:

Under new management, YHOO has taken some increasingly shareholder-friendly steps. It has given up competing with Google in the web search business, a move which is improving free cash flow by reducing capex and operating expenses. It is using the improved cash flow to step up share repurchases (the company bought back more than 7% of its outstanding shares in 2010).YHOO is also taking steps to unlock value from some of its Asian assets in a tax efficient manner, including its 35% stake in publicly traded Yahoo Japan.

Thoughts? Comments? Share 'em below! 

Fool contributor Morgan Housel owns shares of Berkshire Hathaway and Bank of America preferred. Follow him on Twitter @TMFHousel. The Motley Fool owns shares of Wells Fargo. The Fool owns shares of Bank of America and also holds a short position in the stock in a different portfolio. Motley Fool newsletter services have recommended General Motors, Yahoo!, Berkshire Hathaway, and Automatic Data Processing.

Try any of our Foolish newsletter services 
free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.