DS1-13

#include <stdio.h>

#define MAXSIZE 10000

int Max3(int A, int B, int C);

int DivideAndConquer(int List[], int left, int right);

int MaxSubseqSum3(int List[], int N);

int main()
{
    int N;
    int List[MAXSIZE];
    scanf("%d", &N);
    int i;
    for(i=0; i<N; ++i)
        scanf("%d", &List[i]);
    int ans=MaxSubseqSum3(List, N);
    printf("%d\n", ans);
    return 0;
}

int Max3(int A, int B, int C)
{
    return A>B?A>C?A:C:B>C?B:C;
}

int DivideAndConquer(int List[], int left, int right)
{
    int MaxLeftSum, MaxRightSum; /*存放左右子问题的解*/
    int MaxLeftBorderSum, MaxRightBorderSum; /*存放跨分界线的结果*/
    int LeftBorderSum, RightBorderSum;
    int center, i;

    if(left==right)              /*递归的终止条件，子列只有1个数字*/
        if(List[left]>0)
            return List[left];
        else
            return 0;

    /*下面是分的过程*/
    center=(left+right)/2;
    MaxLeftSum=DivideAndConquer(List, left, center);
    MaxRightSum=DivideAndConquer(List, center+1, right);

    MaxLeftBorderSum=0, LeftBorderSum=0;
    for(i=center; i>=left; --i)
    {
        LeftBorderSum+=List[i];
        if(LeftBorderSum>MaxLeftBorderSum)
            MaxLeftBorderSum=LeftBorderSum;
    }

    MaxRightBorderSum=0, RightBorderSum=0;
    for(i=center+1; i<=right; ++i)
    {
        RightBorderSum+=List[i];
        if(RightBorderSum>MaxRightBorderSum)
            MaxRightBorderSum=RightBorderSum;
    }

    return Max3(MaxLeftSum, MaxRightSum, MaxLeftBorderSum+MaxRightBorderSum);
}

int MaxSubseqSum3(int List[], int N)
{
    return DivideAndConquer(List, 0, N-1);
}