优化gcd

int gcd(int x,int y )

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{

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if(x < y) return gcd(y,x); // x>y

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if( y == 0) return x; // if y=0, x is GCD

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else

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{

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if( !(x%2) )

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{

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if( !(y%2) ) //x,y both even

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return 2*gcd(x >> 1, y >> 1);

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else // x is even, y is odd

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return gcd(x >> 1, y );

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}

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else

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{

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if( !(y%2) ) // x is odd, y is even

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return gcd(x, y >> 1);

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else // x, y both odd

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return gcd(y,x-y);

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}

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}

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}