poj2479

最近一阶段写了dp的专题，这是第一题。

题目大意：给定一段数，要求你求出两段和最大的。。

思路：直接两遍的最大和子序列，前面一遍，后面一遍。。然后加起来。。

/*
  Author:yzcstc
  Time:2013.2.26
  State:AC
*/
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <algorithm>
using namespace std;
const int minn = -500000001;
int T , n , a[100005], lsum[3][100005], rsum[3][100005] , ans;

void dp(){
      ans = minn;
      scanf("%d",&n);
      lsum[0][0] = lsum[1][0] = minn;
      rsum[0][n + 1] = rsum[1][n + 1] = minn;
      for (int i = 1; i <= n; ++i)
        {
            scanf("%d",&a[i]);
            lsum[0][i] = max(lsum[0][i - 1] + a[i] , a[i]);
            lsum[1][i] = max(lsum[1][i - 1] , lsum[0][i - 1]);
        }

      for (int i = n; i >= 1; --i)
         {

            rsum[0][i] = max(rsum[0][i + 1] + a[i] , a[i]);
            rsum[1][i] = max(rsum[1][i + 1] , rsum[0][i + 1]);
         }
      for (int i = 2; i <= n; ++i)
            ans = max( ans , 
            max(lsum[0][i - 1], lsum[1][i - 1]) + max( rsum[0][i] , rsum[1][i]));
}

int main(){
      freopen("poj2479.in","r",stdin);
      freopen("poj2479.out","w",stdout);
      scanf("%d",&T);
      for (int i = 1 ; i <= T; ++i)
        {
             dp();
             printf("%d\n",ans);
        }
      fclose(stdin); fclose(stdout);
}