51Nod 1050 循环数组最大子段和 dp

51Nod 1050 循环数组最大子段和 传送门

emmmmm……还以为是数组复制一遍求最大子段和……然额……并不是……想不太到逆向思维的感觉……

循环数组的最大子段和转化成非循环数组可能有两种，一种是非循环数组的最大子段和（大概就是那种连续的，不跨越1和n那种），另一种是sum-最小子段和。

#include<iostream>  
#include<algorithm>
#include<string>
#include<string.h>
typedef long long ll;
using namespace std;
ll n,res,a[50005],b[50005],crt;
ll dp,dpp,sum;
int main()
{
    while (scanf("%lld", &n) != EOF)
    {
        sum = 0;  crt = 0;
        ll minn = 0x3f3f3f3f,maxx=-0x3f3f3f3f;
        for (int i = 1; i <= n; i++)
        {
            scanf("%lld", &a[i]);
            sum += a[i];
            b[i] = a[i];
            if (a[i] < 0) crt++;
        }
        if (crt == n) break;
        for (int i = 1; i <= n; i++)
        {
            dp += a[i];
            dpp+=b[i];
            if (dp >maxx)
                maxx = dp;
            if(dpp<minn)
                minn=dpp;
            if (dp < 0)
                dp = 0;
            if(dpp>0) 
                dpp=0;
        }
        printf("%lld
", max(sum - minn,maxx));
    }
    return 0;
}