【81.82%】【codeforces 740B】Alyona and flowers

time limit per test2 seconds
memory limit per test256 megabytes
inputstandard input
outputstandard output
Little Alyona is celebrating Happy Birthday! Her mother has an array of n flowers. Each flower has some mood, the mood of i-th flower is ai. The mood can be positive, zero or negative.

Let’s define a subarray as a segment of consecutive flowers. The mother suggested some set of subarrays. Alyona wants to choose several of the subarrays suggested by her mother. After that, each of the flowers will add to the girl’s happiness its mood multiplied by the number of chosen subarrays the flower is in.

For example, consider the case when the mother has 5 flowers, and their moods are equal to 1,  - 2, 1, 3,  - 4. Suppose the mother suggested subarrays (1,  - 2), (3,  - 4), (1, 3), (1,  - 2, 1, 3). Then if the girl chooses the third and the fourth subarrays then:

the first flower adds 1·1 = 1 to the girl’s happiness, because he is in one of chosen subarrays,
the second flower adds ( - 2)·1 =  - 2, because he is in one of chosen subarrays,
the third flower adds 1·2 = 2, because he is in two of chosen subarrays,
the fourth flower adds 3·2 = 6, because he is in two of chosen subarrays,
the fifth flower adds ( - 4)·0 = 0, because he is in no chosen subarrays.
Thus, in total 1 + ( - 2) + 2 + 6 + 0 = 7 is added to the girl’s happiness. Alyona wants to choose such subarrays from those suggested by the mother that the value added to her happiness would be as large as possible. Help her do this!

Alyona can choose any number of the subarrays, even 0 or all suggested by her mother.

Input
The first line contains two integers n and m (1 ≤ n, m ≤ 100) — the number of flowers and the number of subarrays suggested by the mother.

The second line contains the flowers moods — n integers a1, a2, …, an ( - 100 ≤ ai ≤ 100).

The next m lines contain the description of the subarrays suggested by the mother. The i-th of these lines contain two integers li and ri (1 ≤ li ≤ ri ≤ n) denoting the subarray a[li], a[li + 1], …, a[ri].

Each subarray can encounter more than once.

Output
Print single integer — the maximum possible value added to the Alyona’s happiness.

Examples
input
5 4
1 -2 1 3 -4
1 2
4 5
3 4
1 4
output
7
input
4 3
1 2 3 4
1 3
2 4
1 1
output
16
input
2 2
-1 -2
1 1
1 2
output
0
Note
The first example is the situation described in the statements.

In the second example Alyona should choose all subarrays.

The third example has answer 0 because Alyona can choose none of the subarrays.

【题目链接】:http://codeforces.com/contest/740/problem/B

【题解】

一开始被题目主体的分析搞晕了；
其实就是把所选择的区间里面的元素全部加在一起；
显然，如果一个区间里面；所有元素的和为负数。肯定不能选的；
因为选择以后答案只会减小；
因为 可能全都不选，所以答案可能为0；

【完整代码】

#include <bits/stdc++.h>
#define LL long long
using namespace std;

const int MAXN = 200;

LL n,m,a[MAXN],sum[MAXN];

int main()
{
   // freopen("F:\rush.txt","r",stdin);
    cin >> n >> m;
    for (int i = 1;i <= n;i++)
        cin >> a[i];
    for (int i = 1;i <= n;i++)
        sum[i] = sum[i-1]+a[i];
    LL ans = 0;
    for (int i = 1;i <= m;i++)
    {
        int l,r;
        cin >> l >> r;
        int dd = sum[r]-sum[l-1];
        if (dd >0)
            ans += dd;
    }
    cout << ans << endl;
    return 0;
}