LeetCode: Surrounded Regions

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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 XX O O XX X O XX O X X

After running your function, the board should be:

X X X XX X X XX X X XX O X X
地址：https://oj.leetcode.com/problems/surrounded-regions/
演算法：我們必須明確一個事實：所有沒有被‘X‘環繞的‘O‘必定有一個從邊緣的‘O’跟它聯通著，所以我的演算法就是從四個邊緣為‘O‘的位置開始往內部按BFS的思想將‘O’設定為‘X’。代碼：

 1 class Solution { 2 public: 3     void solve(vector<vector<char>> &board) { 4         if(board.empty())   return; 5         int m = board.size(); 6         vector<vector<bool> > flag; 7         for(int i = 0; i < m; ++i){ 8             int mi = board[i].size(); 9             flag.push_back(vector<bool>(mi,false));10         }11         for(int i = 0; i < board[0].size(); ++i){12             if(board[0][i] == ‘O‘ && flag[0][i] == false){13                14                 solveCore(board,flag,0,i);15             }16         }17         for(int i = 1; i < m; ++i){18             int mi = board[i].size();19             if(mi > 0 && board[i][0] == ‘O‘ && flag[i][0] == false){20                 21                 solveCore(board,flag,i,0);22             }23             if(mi > 0 && board[i][mi-1] == ‘O‘ && flag[i][mi-1] == false){24                 25                 solveCore(board,flag,i,mi-1);26             }27         }28         for(int i = 1; i < board[m-1].size()-1; ++i){29             if(board[m-1][i] == ‘O‘ && flag[m-1][i] == false){30                 31                 solveCore(board,flag,m-1,i);32             }33         }34         for(int i = 0; i < m; ++i){35             int mi = board[i].size();36             for(int j = 0; j < mi; ++j){37                 if(board[i][j] == ‘O‘ && !flag[i][j])38                     board[i][j] = ‘X‘;39             }40         }41     }42     void solveCore(vector<vector<char> > &board, vector<vector<bool> > &flag, int x, int y){43         queue<pair<int,int> > Q;44         Q.push(pair<int,int>(x,y));45         flag[x][y] = true;46         while(!Q.empty()){47             pair<int,int> pa = Q.front();48             int i = pa.first, j = pa.second;49             if(i > 0 && j < board[i-1].size() && board[i-1][j] == ‘O‘ && flag[i-1][j] == false){50                 flag[i-1][j] = true;51                 Q.push(pair<int,int>(i-1,j));52             }53             if(j < board[i].size()-1 && board[i][j+1] == ‘O‘ && flag[i][j+1] == false){54                 flag[i][j+1] = true;55                 Q.push(pair<int,int>(i,j+1));56             }57             if(i < board.size()-1 && j < board[i+1].size() && board[i+1][j] == ‘O‘ && flag[i+1][j] == false){58                 flag[i+1][j] = true;59                 Q.push(pair<int,int>(i+1,j));60             }61             if(j > 0 && board[i][j-1] == ‘O‘ && flag[i][j-1] == false){62                 flag[i][j-1] = true;63                 Q.push(pair<int,int>(i,j-1));64             }65             Q.pop();66         }67     }68 };

 

LeetCode: Surrounded Regions