Leetcode: Surrounded regions

Given a 2D board containing 'X' and 'O', capture all regions surrounded by 'X'.

A region is captured by flipping all 'O's into 'X's in that surrounded region .

For example,

X X X X

X O O X

X X O X

X O X X

After running your function, the board should be:

X X X X

X X X X

X X X X

X O X X

---

我的最初思路是直接找被包围的O区域，如果这个区域不与边界接触则是被包围区域，但是这种判断要等到dfs完成才能知道是否是被包围区域，所以需要再一次dfs来将这些点变成'X'。所以这样的复杂度太高，Judge Large超时。这个版本的代码包括下面的dfs和changeBoard，以及Solve中被注释掉的部门代码；后来参考了网上的一个代码，将思路转变为先对不被包围的区域做标记'C'，然后再遍历一遍按要求修改。这样就避免了做两遍深度搜索。

代码如下：（有点难看，见谅……）

  1 //

  2 //  SurroundedRegions.cpp

  3 //  SJMcode

  4 //

  5 //  Created by Jiamei Shuai on 13-8-30.

  6 //  Copyright (c) 2013年 Jiamei Shuai. All rights reserved.

  7 //

  8 

  9 

 10 #include <iostream>

 11 #include <vector>

 12 #include <assert.h>

 13 using namespace std;

 14 

 15 class Solution {

 16 public:

 17     

 18     void dfs2(vector<vector<char>> &board, int i, int j)

 19     {

 20         if(i > board.size()-1 || i < 0 || j > board[0].size()-1 || j < 0)

 21             return;

 22         

 23         if(board[i][j] == 'O')

 24         {

 25             board[i][j] = 'C';

 26             dfs2(board, i+1, j);

 27             dfs2(board, i-1, j);

 28             dfs2(board, i, j-1);

 29             dfs2(board, i, j+1);

 30         }

 31             

 32 

 33     }

 34     

 35     bool dfs(vector<vector<char>> &board, int i, int j, int height, int width, vector<vector<int>> &isVis, bool &flag) // working but too slow

 36     {

 37         //if(board[i][j] == 'X')  return true;

 38         

 39         if(i == height-1 || i == 0 || j == width-1 || j == 0)

 40             flag = false; // 'o' touch the border: means this block cannot be the answer

 41         

 42         isVis[i][j] = 1;

 43         

 44         

 45         if(i >= 1 && !isVis[i-1][j] && board[i-1][j] == 'O')

 46         {

 47             dfs(board, i-1, j, height, width, isVis, flag); //上

 48         }

 49         if(i < (int)board.size()-1 && !isVis[i+1][j] && board[i+1][j] == 'O')

 50         {

 51             dfs(board, i+1, j, height, width, isVis, flag); //下

 52         }

 53         if(j >= 1 && !isVis[i][j-1] && board[i][j-1] == 'O')

 54         {

 55             dfs(board, i, j-1, height, width, isVis, flag); //左

 56         }

 57         if(j < (int)board[0].size() && !isVis[i][j+1] && board[i][j+1] == 'O')

 58         {

 59             dfs(board, i, j+1, height, width, isVis, flag); //右

 60         }

 61         

 62         return flag;

 63     }

 64     

 65     void changeBoard(vector<vector<char>> &board, int i, int j) // working but too slow

 66     {

 67         vector<int> queue;

 68         board[i][j] = 'X';

 69         assert(i > 0 && i < board.size()-1 && j > 0 && j < board[0].size()-1);

 70         if(board[i-1][j] == 'O')

 71             changeBoard(board, i-1, j);

 72         if(board[i+1][j] == 'O')

 73             changeBoard(board, i+1, j);

 74         if(board[i][j-1] == 'O')

 75             changeBoard(board, i, j-1);

 76         if(board[i][j+1] == 'O')

 77             changeBoard(board, i, j+1);

 78     }

 79     

 80     void solve(vector<vector<char>> &board) {

 81         // Start typing your C/C++ solution below

 82         // DO NOT write int main() function

 83         int height = (int)board.size();

 84         if(height == 0) return;

 85         int width = (int)board[0].size();

 86         

 87 //        vector<int> temp(width, 0);

 88 //        

 89 //        vector<vector<int>> isVis(height, temp);

 90 //        bool flag = true;

 91 //        

 92 //        for(int i = 0; i < height; i++)

 93 //        {

 94 //            for(int j = 0; j < width; j++)

 95 //            {

 96 //                if(board[i][j] == 'O' && !isVis[i][j])

 97 //                {

 98 //                    flag = true;

 99 //                    if(dfs(board, i, j, height, width, isVis, flag)) // surround regions

100 //                    {

101 //                        changeBoard(board, i, j);  // Find surround region directly may cause runtime error

102 //                    }

103 //                }

104 //            }

105 //        }

106         

107         // Change my strategy to mark unsurrounded regions

108         

109         for(int i = 0; i < height; i++)

110         {

111             dfs2(board, i, 0);

112             dfs2(board, i, width-1);

113         }

114         

115         for(int j = 0; j < width; j++)

116         {

117             dfs2(board, 0, j);

118             dfs2(board, height-1, j);

119         }

120         

121         for(int i = 0; i < height; i++)

122         {

123             for(int j = 0; j < width; j++)

124             {

125                 if(board[i][j] == 'O') board[i][j] = 'X';

126                 if(board[i][j] == 'C') board[i][j] = 'O';

127             }

128         }

129         

130         // print result

131         for(int i = 0; i < height; i++)

132         {

133             for(int j = 0; j < width; j++)

134             {

135                 cout << board[i][j] << ' ';

136             }

137             cout << endl;

138         }

139         

140     }

141     

142 };

143 

144 int main()

145 {

146     vector<vector<char>> board{{'X','X','X','X'},{'X','O','O','X'},{'X','X','O','X'},{'X','O','X','X'}};

147     

148     

149     Solution sln;

150     sln.solve(board);

151     

152     return 0;

153 }

View Code

 此外，提交的代码是不能有输出语句的，否则会报Internal Error。

 BFS也可以做：http://blog.sina.com.cn/s/blog_b9285de20101j1dt.html