SPOJ 274 Johnny and the Watermelon Plantation（TLE）

O(n^3)的时间复杂度，改了半天交了二三十遍，TLE到死，实在没办法了……

跪求指点！！！

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>

using namespace std;

const int MAXN = 1010;
const int INF = 1 << 30;

int mat[MAXN][MAXN];
int SubSum[MAXN][MAXN];
int tempS[MAXN];
int N, K;
int maxI, maxJ;
int hang[MAXN], cntH;
int lie[MAXN], cntL;

//预处理左上角[1,1]右下角[i,j]的所有矩阵和
void init()
{
    memset( SubSum, 0, sizeof(SubSum) );
    for ( int i = 1; i <= maxI; ++i )
    {
        int tmps = 0;
        for ( int j = 1; j <= maxJ; ++j )
        {
            tmps += mat[i][j];
            SubSum[i][j] = SubSum[i - 1][j] + tmps;
        }
    }
    return;
}

void solved()
{
    int ans = INF;
    for ( int i = 0; i < cntH; ++i ) //枚举矩形上边界
    for ( int j = i; j < cntH; ++j )  //枚举矩形下边界
    {
        int stL = hang[i], edL = hang[j];
        tempS[0] = 0;
        int st = 1;
        //扫描法，参考刘汝佳训练指南p.48 LA 2678
        for ( int m = 1; m <= cntL; ++m )
        {
            int k = lie[m - 1];
            tempS[m] = SubSum[edL][k] - SubSum[edL][k-1] - SubSum[stL-1][k] + SubSum[stL-1][k-1];
            tempS[m] += tempS[m - 1];
            if ( tempS[st - 1] > tempS[m] - K ) continue;
            while ( tempS[st] <= tempS[m] - K ) ++st;
            ans = min( ans, ( lie[m - 1] - lie[st - 1] ) * ( edL - stL ) );
        }
    }
    printf( "%d
", ans );
    return;
}

int main()
{
    //freopen( "in.txt", "r", stdin );
    int T;
    scanf( "%d", &T );
    while ( T-- )
    {
        memset( mat, 0, sizeof(mat) );
        scanf( "%d%d", &N, &K );
        maxI = 0;
        maxJ = 0;
        cntL = 0, cntH = 0;
        bool flag = false;
        for ( int i = 0; i < N; ++i )
        {
            int x, y, f;
            scanf("%d%d%d", &x, &y, &f );
            hang[cntH++] = x;  //行列离散化
            lie[cntL++] = y;
            maxI = max( maxI, x );
            maxJ = max( maxJ, y );
            mat[x][y] = f;
            if ( f >= K )  //特殊情况，只覆盖一个点
            {
                flag = true;
            }
        }

        if ( N == 1 || flag )
        {
            puts("0");
            continue;
        }

        sort( hang, hang + cntH );
        sort( lie, lie + cntL );
        cntH = unique( hang, hang + cntH ) - hang;
        cntL = unique( lie, lie + cntL ) - lie;

        init();
        solved();
    }
    return 0;
}