蓝桥杯：合并石子（区间DP+平行四边形优化）

http://lx.lanqiao.cn/problem.page?gpid=T414

题意：……

思路：很普通的区间DP，但是因为n<=1000，所以O（n^3）只能拿90分。上网查了下了解了平行四边形优化：地址。

但是看不懂。

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const LL INF = 100000000000000000LL;
LL dp[1010][1010], s[1010][1010];
LL sum[1010];
int main() {
    LL ans = 0;
    int n; scanf("%d", &n);
    for(int i = 1; i <= n; i++) scanf("%lld", &dp[i][i]), sum[i] = sum[i-1] + dp[i][i], dp[i][i] = 0, s[i][i] = i;
    for(int len = 1; len < n; len++) {
        for(int l = 1; l + len <= n; l++) {
            int r = l + len;
            dp[l][r] = INF;
            for(int k = s[l][r-1]; k <= s[l+1][r]; k++) {
                LL now = dp[l][k] + dp[k+1][r] + sum[r] - sum[l - 1];
                if(dp[l][r] > now) dp[l][r] = now, s[l][r] = k;
            }
        }
    }
    printf("%lld
", dp[1][n]);
    return 0;
}