poj 2479 Maximum sum（DP）

题意：求两段连续和使之最大

思路：两个方向都处理。先正方向求出到每个index的连续最大和，然后从前往后把每个位置的最大和都更新一下（更新为前边所有和中最大的那个），反方向做同样的处理。

然后i 1..n枚举

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<queue>
#include<algorithm>
#include<map>
#include<iomanip>
#include<climits>
#include<string.h>
#include<numeric>
#include<cmath>
#include<stdlib.h>
#include<vector>
#include<stack>
#include<set>
#define INF 1e7
#define MAXN 100010
#define maxn 1000010
#define Mod 1000007
#define N 1299800
using namespace std;
typedef long long LL;

int T;
int n, num[50010];
int sum1[50010],sum2[50010];

void run()
{
    scanf("%d", &n);
    memset(sum1, 0, sizeof(sum1));
    memset(sum2, 0, sizeof(sum2));
    memset(num, 0, sizeof(num));
    for (int i = 1; i <= n; ++i) 
        scanf("%d",&num[i]);
    if (n == 2) {
        printf("%d
",num[1] + num[2]);
        return;
    }
    for (int i = 1; i <= n; ++i) {
        if (sum1[i - 1] > 0)
            sum1[i] = sum1[i - 1] + num[i];
        else sum1[i] = num[i];
    }
    for (int i = 1; i <= n; ++i)
        sum1[i] = max(sum1[i - 1], sum1[i]);
    for (int i = n; i >= 1; --i) {
        if (sum2[i + 1] > 0)
            sum2[i] = sum2[i + 1] + num[i];
        else sum2[i] = num[i];
    }
    for (int i = n; i >= 1; --i)
        sum2[i] = max(sum2[i],sum2[i+1]);
    int ans = -INF;
    for (int i = 1; i <= n; ++i)
        ans = max(ans, sum1[i] + sum2[i + 1]);
    printf("%d
",ans);
}

int main()
{
    scanf("%d", &T);
    while (T--)
        run();
    return 0;
}