[Leetcode] Surrounded regions @ Python

Original title address: https://oj.leetcode.com/problems/surrounded-regions/

Test instructions

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

A region was captured by flipping all ‘O‘ s into ‘X‘ s-surrounded region.

For example,

x x x xx o o xx x o xx o x x

After running your function, the board should is:

x x x xx x x xx x x xx O x x

Problem solving ideas: This problem can be solved using BFS and DFS two ways. DFS will time out. BFS can be AC. Start searching from the side, if ' o ', then search for the elements around ' o ' and replace ' o ' with ' D ', so that each side is DFS or BFS again. and the inner ' O ' is not going to change. So down, not surrounded by the ' o ' are all replaced by ' D ', surrounded by ' o ' or ' o ', unchanged. Then iterate over, replace ' o ' with ' X ' and ' D ' into ' o '.

DFS code, because the problem of recursion depth will explode:

classSolution:#@param board, a 9x9 2D array    #Capture all regions by modifying the input board In-place.    #Don't return any value.    defSolve (self, board):defdfs (x, y):ifX<0orX>m-1orY<0orY>n-1orboard[x][y]!='O':returnBoard[x][y]='D'DFS (x-1, y) dfs (x+1, y) dfs (x, y+1) Dfs (x, y-1)                ifLen (board) = = 0:returnm= Len (board); n =Len (board[0]) forIinchRange (m): DFS (i, 0); Dfs (i, n-1)         forJinchRange (1, n-1): Dfs (0, J); DFS (M-1, J) forIinchRange (m): forJinchrange (n):ifBOARD[I][J] = ='O': board[i][j] = ='X'                elifBOARD[I][J] = ='D': board[i][j] = ='O'

BFS Code:

classSolution:#@param board, a 2D array    #Capture all regions by modifying the input board In-place.    #Don't return any value.    defSolve (self, board):#The ' O ' on the outermost four edges of board is certainly not surrounded by ' X ', starting with the ' O ', using BFS to find all the neighboring ' o ', and changing these ' o ' to another symbol, such as ' D '. Then scan the board again and replace ' o ' with ' X ' and ' D ' with ' o '.         defFill (x, y):ifX<0orX>m-1orY<0orY>n-1orBoard[x][y]! ='O':returnQueue.append ((x, y)) Board[x][y]='D'                    defBFS (x, y):ifboard[x][y]=='O': Fill (x, y) whileQueue:curr=queue.pop (0); I=CURR[0]; J=curr[1] Fill (i+1,J); Fill (i-1,j); fill (i,j+1); Fill (i,j-1)                        ifLen (board) ==0:returnm=len (board); N=len (Board[0]); Queue=[]         forIinchrange (N): BFs (0,i); BFS (M-1, i) forJinchRange (1, m-1): BFS (j,0); BFS (J,n-1)         forIinchRange (m): forJinchrange (n):ifBOARD[I][J] = ='D': board[i][j] ='O'                elifBOARD[I][J] = ='O': board[i][j] ='X'

Reference:

This article references [1] but made a small change. The original BFS function has one more of the following: Queue.append (x, y) is a repetition of the fill function. The result is a double backup for each element of the queue.

[1] http://www.cnblogs.com/zuoyuan/p/3765434.html

[Leetcode] Surrounded regions @ Python