130. Surrounded Regions

摘要： Given a 2D board containing 'X' and 'O' (the letter O), capture all regions surrounded by 'X'. A region is captured ...

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

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

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

Explanation:

Surrounded regions shouldn’t be on the border, which means that any 'O' on the border of the board are not flipped to 'X'. Any 'O' that is not on the border and it is not connected to an 'O' on the border will be flipped to 'X'. Two cells are connected if they are adjacent cells connected horizontally or vertically.

難度：medium

題目：給定由X和O組成的二維表格，找出所有由X包含的區域並將O轉成X.

思路：遍歷二維表格四邊將以0開始的元素延伸到整個二維表並將其值置成非X非O（如Ｂ）字元用以區分與其它元素。然後遍歷整個二維表將除Ｂ以外的所有元素設為X, Ｂ元素恢復為O

Runtime: 4 ms, faster than 96.33% of Java online submissions for Surrounded Regions.

Memory Usage: 40.7 MB, less than 100.00% of Java online submissions for Surrounded Regions.

class Solution {
public void solve(char[][] board) {
if (null == board || board.length <= 1 || board[0].length <= 1) {
return;
}

int m = board.length, n = board[0].length;
for (int i: Arrays.asList(0, m - 1)) {
for (int j = 0; j < n; j++) {
if (board[i][j] == 'O') {
color(board, i, j);
}
}
}
for (int i: Arrays.asList(0, n - 1)) {
for (int j = 0; j < m; j++) {
if (board[j][i] == 'O') {
color(board, j, i);
}
}
}

for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
if (board[i][j] == 'O' || board[i][j] == 'X') {
board[i][j] = 'X';
} else {
board[i][j] = 'O';
}
}
}
}

private void color(char[][] board, int i, int j) {
if (i < 0 || i >= board.length 
|| j < 0 || j >= board[i].length 
|| board[i][j] != 'O') {
return;
}

board[i][j] = 'B';
color(board, i + 1, j);
color(board, i - 1, j);
color(board, i, j + 1);
color(board, i, j - 1);
}
}