《数据结构与算法分析 C语言描述》读书笔记——分治算法

#include <stdio.h>

int imax (int a, int b, int c)
{
    return (a>b?a:b)>c?(a>b?a:b):c;
}

static int MaxSubSum(const 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 = MaxSubSum(A, Left, Center);              //1
    int MaxRightSum = MaxSubSum(A, Center+1, Right);          //2

    int MaxLeftBorderSum = 0, LeftBorderSum = 0;              //3
    for (int i = Center; i >= Left; i--)
    {
        LeftBorderSum += A[i];
        if (LeftBorderSum > MaxLeftBorderSum)
            MaxLeftBorderSum = LeftBorderSum;
    }

    int MaxRightBorderSum = 0, RightBorderSum = 0;
    for (int i = Center+1; i <= Right; i++)
    {
        RightBorderSum += A[i];
        if (RightBorderSum > MaxRightBorderSum)
            MaxRightBorderSum = RightBorderSum;
    }                                                         //4

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

int MaxSubSequenceSum(const int A[], int N)
{
    return MaxSubSum(A, 0, N - 1);
}

int main()
{
    int A[] = {-2, 11, -4, 13, -5, -2};
    printf("%d\n", MaxSubSequenceSum(A, sizeof(A)/sizeof(A[0])));

    return 0;
}