java 之连续子序列最大和问题的四个解法

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import java.util.Random;public final class MaxSumTest{    static private int seqStart = 0;    static private int seqEnd = -1;    /**     *ustczyy  *zyyjiao@mail.ustc.edu.cn     */    public static int maxSubSum1( int [ ] a )    {        int maxSum = 0;        for( int i = 0; i < a.length; i++ )            for( int j = i; j < a.length; j++ )            {                int thisSum = 0;                for( int k = i; k <= j; k++ )                    thisSum += a[ k ];                if( thisSum > maxSum )                {                    maxSum   = thisSum;                    seqStart = i;                    seqEnd   = j;                }            }        return maxSum;    }    /**     *ustczyy  *zyyjiao@mail.ustc.edu.cn     */    public static int maxSubSum2( int [ ] a )    {        int maxSum = 0;        for( int i = 0; i < a.length; i++ )        {            int thisSum = 0;            for( int j = i; j < a.length; j++ )            {                thisSum += a[ j ];                if( thisSum > maxSum )                {                    maxSum = thisSum;                    seqStart = i;                    seqEnd   = j;                }            }        }        return maxSum;    }    /**     *ustczyy  *zyyjiao@mail.ustc.edu.cn     */    public static int maxSubSum3( int [ ] a )    {        int maxSum = 0;        int thisSum = 0;        for( int i = 0, j = 0; j < a.length; j++ )        {            thisSum += a[ j ];            if( thisSum > maxSum )            {                maxSum = thisSum;                seqStart = i;                seqEnd   = j;            }            else if( thisSum < 0 )            {                i = j + 1;                thisSum = 0;            }        }        return maxSum;    }    /**     * *ustczyy  *zyyjiao@mail.ustc.edu.cn     */    private static int maxSumRec( int [ ] a, int left, int right )    {        int maxLeftBorderSum = 0, maxRightBorderSum = 0;        int leftBorderSum = 0, rightBorderSum = 0;        int center = ( left + right ) / 2;        if( left == right )  // Base case            return a[ left ] > 0 ? a[ left ] : 0;        int maxLeftSum  = maxSumRec( a, left, center );        int maxRightSum = maxSumRec( a, center + 1, right );        for( int i = center; i >= left; i-- )        {            leftBorderSum += a[ i ];            if( leftBorderSum > maxLeftBorderSum )                maxLeftBorderSum = leftBorderSum;        }        for( int i = center + 1; i <= right; i++ )        {            rightBorderSum += a[ i ];            if( rightBorderSum > maxRightBorderSum )                maxRightBorderSum = rightBorderSum;        }        return max3( maxLeftSum, maxRightSum,                     maxLeftBorderSum + maxRightBorderSum );    }    /**     * Return maximum of three integers.     */    private static int max3( int a, int b, int c )    {        return a > b ? a > c ? a : c : b > c ? b : c;    }    /**     * ustczyy  *zyyjiao@mail.ustc.edu.cn     */    public static int maxSubSum4( int [ ] a )    {        return a.length > 0 ? maxSumRec( a, 0, a.length - 1 ) : 0;    }    public static void getTimingInfo( int n, int alg )    {        int [] test = new int[ n ];        long startTime = System.currentTimeMillis( );;        long totalTime = 0;        int i;        for( i = 0; totalTime < 4000; i++ )        {            for( int j = 0; j < test.length; j++ )                test[ j ] = rand.nextInt( 100 ) - 50;            switch( alg )            {              case 1:                maxSubSum1( test );                break;              case 2:                maxSubSum2( test );                break;              case 3:                maxSubSum3( test );                break;              case 4:                maxSubSum4( test );                break;            }            totalTime = System.currentTimeMillis( ) - startTime;        }        System.out.println( "Algorithm #" + alg + "\t"             + "N = " + test.length             + "\ttime = " + ( totalTime * 1000 / i ) + " microsec" );    }         private static Random rand = new Random( );        /**     * Simple test program.     */    public static void main( String [ ] args )    {        int a[ ] = { 4, -3, 5, -2, -1, 2, 6, -2 };        int maxSum;        maxSum = maxSubSum1( a );        System.out.println( "Max sum is " + maxSum + "; it goes"                       + " from " + seqStart + " to " + seqEnd );        maxSum = maxSubSum2( a );        System.out.println( "Max sum is " + maxSum + "; it goes"                       + " from " + seqStart + " to " + seqEnd );        maxSum = maxSubSum3( a );        System.out.println( "Max sum is " + maxSum + "; it goes"                       + " from " + seqStart + " to " + seqEnd );        maxSum = maxSubSum4( a );        System.out.println( "Max sum is " + maxSum );                  // Get some timing info        for( int n = 10; n <= 1000000; n *= 10 )            for( int alg = 4; alg >= 1; alg-- )            {                if( alg == 1 && n > 50000 )                    continue;                    getTimingInfo( n, alg );            }    }}
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