动态规划 三.最大子段和

例如，当a={-2,11,-4,13,-5,-2}时，最大子段和是20。

#include<iostream>
using namespace std;

#define N 100

int maxsum1(int *a,int n)//动态规划算法 
{
    int sum=0,b=0;
    for(int i=1;i<=n;i++)
    {
        if(b>0)b+=a[i];
        else b=a[i];
        if(b>sum)sum=b;
     } 
     return sum;
} 

int maxsubsum(int *a,int left,int right)
{
    int sum=0;
    if(left==right) sum=a[left]>0?a[left]:0;
    else 
    {
        int center=(left+right)/2;
        int leftsum=maxsubsum(a,left,center);
        int rightsum=maxsubsum(a,center+1,right);
        int s1=0;
        int lefts=0;
        for(int i=center;i>=left;i--)
        {
            lefts+=a[i];
            if(lefts>s1)s1=lefts;
        }
        int s2=0;
        int rights=0;
        for(int i=center+1;i<=right;i++)
        {
            rights+=a[i];
            if(rights>s2)s2=rights;
        }
        sum=s1+s2;
        if(sum<leftsum)sum=leftsum;
        if(sum<rightsum)sum=rightsum;
    }
    return sum;
}

int maxsum(int *a,int n)//分治算法 
{
    return maxsubsum(a,1,n); 
}


int main()
{
    int a[N],n;
    cin>>n;
    for(int i=1;i<=n;i++)
    cin>>a[i];
    cout<<"分治算法：" <<maxsum(a,n)<<endl; 
    cout<<"动态规划算法：" <<maxsum1(a,n)<<endl;
    return 0;
}