package a;

/*

 * 最大子序列和问题，时间复杂度O（NlogN)

 */

public class Sequence {

    private static int maxSumRec(int[] a,int left,int right ){

        if(left==right)

            if(a[left]>0)

                return a[left];

            else

                return 0;

        int center=(left+right)/2;

        int maxLeftSum =maxSumRec(a,left,center);//递归求前部分最大值

        int maxRightSum = maxSumRec(a,center+1,right);//递归求后部分最大值

        //求前半部分含最后一个元素的最大值

        int maxLeftBorderSum=0,leftBorderSum=0;

        for(int i=center;i>=left;i--){

            leftBorderSum+=a[i];

            if(leftBorderSum>maxLeftBorderSum)

                maxLeftBorderSum=leftBorderSum;

        }

        //求后半部分含第一个元素的最大值

        int maxRightBorderSum=0,rightBorderSum=0;

        for(int i=center+1;i<=right;i++){

            rightBorderSum+=a[i];

            if(rightBorderSum>maxRightBorderSum)

                maxRightBorderSum=rightBorderSum;

        }

        return max3(maxLeftSum,maxRightSum,maxLeftBorderSum+maxRightBorderSum);

    }

    //求最大值

    private static int max3(int maxLeftSum, int maxRightSum, int q) {

        // TODO Auto-generated method stub

        int max;

        if(maxLeftSum>maxRightSum)

            max=maxLeftSum;

        else

            max=maxRightSum;

        if(q>max)

            max=q;

        return max;

    }

    public static void main(String[] args){

        int[] a={4,-3,5,-2,-1,2,6,-2};

        System.out.println(maxSumRec(a, 0, a.length-1));

    }

}