[HAOI2007]修筑绿化带

这道题要用到“尺取法”（又名“滑动窗口算法”），具体的描述可参考P2716 【和谐的雪花】——AC-Evil的洛谷博客

这道题跟上面的这题几乎一样。

#include <bits/stdc++.h>

using namespace std;

#define re register
#define rep(i, a, b) for (re int i = a; i <= b; ++i)
#define repd(i, a, b) for (re int i = a; i >= b; --i)
#define maxx(a, b) a = max(a, b);
#define minn(a, b) a = min(a, b);
#define LL long long
#define inf (1 << 30)

inline int read() {
    int w = 0, f = 1; char c = getchar();
    while (!isdigit(c)) f = c == '-' ? -1 : f, c = getchar();
    while (isdigit(c)) w = (w << 3) + (w << 1) + (c ^ '0'), c = getchar();
    return w * f;
}

const int maxn = 1e3 + 5;

int N, M, A, B, C, D, s[maxn][maxn], g[maxn][maxn];
int q[maxn], h, t;
#define square(x1, y1, x2, y2) (s[x2][y2]-s[x1-1][y2]-s[x2][y1-1]+s[x1-1][y1-1])

int main() {
    N = read(), M = read(), A = read(), B = read(), C = read(), D = read();
    rep(i, 1, N)
        rep(j, 1, M)
            s[i][j] = s[i-1][j] + s[i][j-1] - s[i-1][j-1] + read();
    rep(i, C, N) {
        h = t = 0;
        rep(x, D, M) {
            while (h < t && x-q[h] >= B-D-1) h++;
            while (h < t && square(i-C+1, x-D+1, i, x) < square(i-C+1, q[t-1]-D+1, i, q[t-1])) t--;
            q[t++] = x;
            g[i][x] = square(i-C+1, q[h]-D+1, i, q[h]);
        }
    }
    int ans = 0;
    rep(i, B-1, M-1) {
        h = t = 0;
        rep(x, C, N-1) {
            while (h < t && x-q[h] >= A-C-1) h++;
            while (h < t && g[x][i] < g[q[t-1]][i]) t--;
            q[t++] = x;
            if (x-A+2>0)
                maxx(ans, square(x-A+2, i-B+2, x+1, i+1) - g[q[h]][i]);
        }
    }

    printf("%d", ans);

    return 0;
}