题意
将一个长度为n的数组划分为a,b,c三段(每一段都可以为空),使得a段和c段的和相等,问a段的和的最大值是多少?
思路
求前缀和,后缀和。然后从大到小枚举后缀,在前缀中查找相等,如果能找到且a,c两段不重叠,那么就是答案。注意开long long
#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5+5;
typedef long long LL;
int n, d[MAXN];
LL pre[MAXN], suf[MAXN];
int main()
{
scanf("%d", &n);
for (int i = 0; i < n; i++) scanf("%d", &d[i]);
pre[0] = d[0];
for (int i = 1; i < n; i++) pre[i] = d[i] + pre[i-1];
suf[n-1] = d[n-1];
for (int i = n-2; i >= 0; i--) suf[i] = d[i] + suf[i+1];
LL ans = 0;
for (int i = 0; i < n; i++)
{
int pos = lower_bound(pre, pre+n, suf[i])-pre;
if (pos < i && pre[pos] == suf[i])
{
ans = max(ans, suf[i]);
break;
}
}
printf("%lld", ans);
return 0;
}
/*
5
1 3 1 1 4
*/