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前言

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一、力扣130. 被围绕的区域

class Solution {boolean[][] flag;int[][] move = {{0,1},{0,-1},{-1,0},{1,0}};public void solve(char[][] board) {flag = new boolean[board.length][board[0].length];for(int i = 0; i < board.length; i ++){if(board[i][0] == 'O' && flag[i][0] == false){dfs(board, i, 0);}if(board[i][board[0].length-1] == 'O' && flag[i][board[0].length-1] == false){dfs(board, i, board[0].length-1);}}for(int i = 0; i < board[0].length; i ++){if(board[0][i] == 'O' && flag[0][i] == false){dfs(board, 0, i);}if(board[board.length-1][i] == 'O' && flag[board.length-1][i] == false){dfs(board, board.length-1, i);}}for(int i = 0; i < board.length; i ++){for(int j = 0; j < board[0].length; j ++){if(board[i][j] == 'O' && flag[i][j] == false){board[i][j] = 'X';}}}}public void dfs(char[][] board, int x, int y){flag[x][y] = true;for(int i = 0; i < 4; i ++){int nextX = x + move[i][0];int nextY = y + move[i][1];if(nextX < 0 || nextX >= board.length || nextY < 0 || nextY >= board[x].length || flag[nextX][nextY] == true || board[nextX][nextY] == 'X'){continue;}dfs(board, nextX, nextY);}}}

二、力扣417. 太平洋大西洋水流问题

class Solution {int[][] move = {{0,1},{0,-1},{-1,0},{1,0}};boolean[][][] flag;// boolean[][] visited;public List<List<Integer>> pacificAtlantic(int[][] heights) {List<List<Integer>> res = new ArrayList<>();int row = heights.length, col = heights[0].length;flag = new boolean[row][col][2];// visited = new boolean[row][col];for(int i = 0; i < row; i ++){if(flag[i][0][0] == false){dfs(heights, i, 0, 0);}if(flag[i][col-1][1] == false){dfs(heights, i, col-1, 1);}}for(int i = 0; i < col; i ++){if(flag[0][i][0] == false){dfs(heights, 0, i, 0);}if(flag[row-1][i][1] == false){dfs(heights, row-1, i, 1);}}for(int i = 0; i < row; i ++){for(int j = 0; j < col; j ++){if(flag[i][j][0] == true && flag[i][j][1] == true){res.add(Arrays.asList(i,j));}}}return res;}public void dfs(int[][] heights, int x, int y, int t){if(t == 0){flag[x][y][0] = true;}else{flag[x][y][1] = true;}for(int i = 0; i < 4; i ++){int nextX = x + move[i][0];int nextY = y + move[i][1];if(nextX < 0 || nextX >= heights.length || nextY < 0 || nextY >= heights[x].length || heights[nextX][nextY] < heights[x][y]){continue;}if(t == 0 && flag[nextX][nextY][0]){continue;}if(t == 1 && flag[nextX][nextY][1]){continue;}dfs(heights, nextX, nextY, t);}}
}

三、力扣827. 最大人工岛

class Solution {int[][] move = {{0,1},{0,-1},{-1,0},{1,0}};int count;public int largestIsland(int[][] grid) {int res = 0;int path = 0;int mark = 1;Map<Integer,Integer> map = new HashMap<>();for(int i = 0; i < grid.length; i ++){for(int j = 0; j < grid[0].length; j ++){if(grid[i][j] == 1){count = 0;mark ++;dfs1(grid, i, j, mark);map.put(mark,count);res = Math.max(res,count);}}}for(int i = 0; i < grid.length; i ++){for(int j = 0; j < grid[0].length; j ++){if(grid[i][j] == 0){Set<Integer> set = new HashSet<>();path = 0;for(int t = 0; t < 4; t ++){int nextX = i + move[t][0];int nextY = j + move[t][1];if(nextX < 0 || nextX >= grid.length || nextY < 0 || nextY >= grid[0].length ||grid[nextX][nextY] == 0){continue;}set.add(grid[nextX][nextY]);}for(Integer in : set){path += map.get(in);}path += 1;res = Math.max(res, path);}}}return res;}public void dfs1(int[][] grid, int x, int y, int mark){count ++;grid[x][y] = mark;for(int i = 0; i < 4; i ++){int nextX = x + move[i][0];int nextY = y + move[i][1];if(nextX < 0 || nextX >= grid.length || nextY < 0 || nextY >= grid[0].length || grid[nextX][nextY] == 0 || grid[nextX][nextY] == mark){continue;}dfs1(grid, nextX, nextY, mark);}}
}