Question

我目前正在参加USACO 2006大赛的练习题，基本上如下：

给定点和X的网格，例如
......
..X..X
X..X..
......
..X...
找到面积最大的矩形，使其周长为矩形不包含X.例如，矩形表示 f是最好的可能性。
.ffff.
.fX.fX
Xf.Xf.
.ffff.
..X...

Here是最初的问题陈述。

到目前为止，我已经编写了一个代码，该代码在13个测试用例中有11个以正确的输出运行，但最后两个测试用例中的两个不起作用。这是我的代码。

#include <iostream>
using namespace std;

#define INF 0x3f3f3f3f

const int MAX = 200 + 5;
int N, M, MAXAREA = 0;
bool ok[MAX][MAX][MAX], land[MAX][MAX];

int main() {
  /// read in input
  // land[i][j] = 1 if there is a swampy area at (i, j)
  cin >> N >> M;
  for(int i = 0; i < N; i++) {
    for(int j = 0; j < M; j++) {
      char ch;
      cin >> ch;
      land[i][j] = (ch == 'X');
    }
  }

  /// precompute crossing lines
  // ok[col][row1][row2]: there are not X's from row1 to row2 in column col
  for(int col = 0; col < M; col++)
    for(int row1 = 0; row1 < N; row1++)
      for(int row2 = row1; row2 < N; row2++) {
        if(land[row2][col]) break;
        else ok[col][row1][row2] = true;
      }

  /// run dynamic programming
  // first loop through all pairs of rows
  for(int row1 = 0; row1 < N; row1++) {
    for(int row2 = row1; row2 < N; row2++) {
      // for each pair of rows, go from left to right for columns. lhs_col = LHS column
      for(int lhs_col = 0; lhs_col < M; ) {
        // find the first LHS column that has no X's at all
        if(!ok[lhs_col][row1][row2])  {
          lhs_col++;
          continue;
        }
        // next_col & last_col are next & last columns that are perfectly clear 
        int cur_col = lhs_col, next_col = INF, last_col = lhs_col;
        // make sure cur_col has top and bottom vertices clear
        while(cur_col < M && !land[row1][cur_col] && !land[row2][cur_col]) {
          // finds the first and last column that are perfectly clear
          if(ok[cur_col][row1][row2])
            next_col = min(next_col, last_col = cur_col);
          cur_col++;
        }
        // get maximum area
        MAXAREA = max(MAXAREA, (row2 - row1 + 1) * (last_col - lhs_col + 1));
        // if there is no next_col that is perfectly clear or the next one is itself
        if(next_col == INF || next_col == lhs_col) break;
        else lhs_col = next_col;
      }
    }
  }

  /// output answer
  cout << MAXAREA << endl;
}

而here是导致其无效的输入案例之一。我的代码输出966，而它应该是1488.另一个测试用例是如此之大，没有必要在这里粘贴它。我非常感谢任何帮助。非常感谢你！

USACO 2006 FortMoo DP Bug

0 个答案: