标签：style blog http color os art
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
题目不难理解，就是把被X包围的O变成X，从题目给出的例子可以看出，只有在边界有O的地方，才不会被X包围，
故将起始搜索位置缩小为边界，常用的搜索方法为深搜和广搜
最开始题目采用深搜去解决发现题目ac不了，深搜的最大深度可能为n*m，但空间复杂度为O(1)
下面提供深搜代码
int dx[] = {-1,0,1,0}; int dy[] = {0,1,0,-1}; class Solution { public: typedef vector<vector<char> > Board; void dfs(int x, int y, Board& board){ if(x < 0 || x >= board.size() || y < 0 || y >=board[0].size() || board[x][y]!=‘O‘) return; board[x][y] = ‘Y‘; for(int i = 0 ; i < 4; ++ i){ dfs(x+dx[i],y+dy[i],board); } } void solve(vector<vector<char> > &board){ if(board.size() == 0) return; int n = board.size(),m = board[0].size(); for(int i = 0 ; i < n; ++ i){ dfs(i,0,board); dfs(i,m-1,board); } for(int j = 0; j < m; ++ j){ dfs(0,j,board); dfs(n-1,j,board); } for(int i = 0 ; i < n ; ++ i){ for(int j = 0 ; j < m ; ++ j){ board[i][j]=(board[i][j]==‘Y‘?‘O‘:‘X‘); } } }
故本题用广搜去解决，广搜的最大深度为max(n,m)，但要开辟一个queue，故其空间复杂度为O(max(n,m))
int dx[] = {-1,0,1,0}; int dy[] = {0,1,0,-1}; typedef pair<int,int> Point; typedef vector<vector<char> > Board; queue<Point> que; void bfs(Board& board){ while(!que.empty()){ Point p = que.front();que.pop(); board[p.first][p.second]=‘Y‘; for(int i = 0 ; i < 4; ++ i){ int newX = x+dx, newY = y+dy; if(newX>=0 && newX<board.size() && newY>=0 && newY< board.size() && board[newX][newY]==‘O‘ ) que.push(Point(newX,newY)); } } } void solve(vector<vector<char> >& board){ if(board.size() == 0) return; int n = board.size(),m = board[0].size(); for(int i = 0 ; i < n; ++ i){ if(board[i][0] == ‘O‘) que.push(Point(i,0)); if(board[i][m-1] == ‘O‘) que.push(Point(i,m-1)); } bfs(board); for(int j = 0; j < m; ++ j){ if(board[0][j] == ‘O‘) que.push(Point(0,j)); if(board[n-1][j] == ‘O‘) que.push(Point(n-1,j)); } bfs(board); for(int i = 0 ; i < n ; ++ i){ for(int j = 0 ; j < m ; ++ j){ board[i][j]=(board[i][j]==‘Y‘?‘O‘:‘X‘); } } }
Leetcode Surrounded Regions,布布扣,bubuko.com
标签：style blog http color os art
原文地址：http://www.cnblogs.com/xiongqiangcs/p/3797135.html
