Given a 2d grid map of ‘1‘s (land) and ‘0‘s (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
Example 1:
Input: 11110 11010 11000 00000 Output: 1
Example 2:
Input: 11000 11000 00100 00011 Output: 3
Solution 1:
Time: O(M * N)
class Solution { public int numIslands(char[][] grid) { if (grid == null || grid.length == 0 || grid[0].length == 0) { return 0; } int row = grid.length; int col = grid[0].length; int res = 0; for (int i = 0; i < row; i++) { for (int j = 0; j < col; j++) { if (grid[i][j] == ‘1‘) { floodFill(grid, i, j); res += 1; } } } return res; } private void floodFill(char[][] grid, int i, int j) { // need to include grid[i][j] == 0 in base case if (i < 0 || i >= grid.length || j < 0 || j >= grid[0].length || grid[i][j] == ‘0‘) { return; } grid[i][j] = ‘0‘; floodFill(grid, i - 1, j); floodFill(grid, i + 1, j); floodFill(grid, i, j - 1); floodFill(grid, i, j + 1); } }
solution 2:
class Solution { private int row; private int col; public int numIslands(char[][] grid) { if (grid == null || grid.length == 0 || grid[0].length == 0) { return 0; } row = grid.length; col = grid[0].length; int res = 0; for (int i = 0; i < row; i++) { for (int j = 0; j < col; j++) { if (grid[i][j] == ‘1‘) { floodFillBFS(grid, i, j); res += 1; } } } return res; } private void floodFillBFS(char[][] grid, int i, int j) { grid[i][j] = ‘0‘; Queue<Integer> queue = new LinkedList<>(); queue.offer(i * col + j); while (!queue.isEmpty()) { int cur = queue.poll(); int m = cur / col; int n = cur % col; if (m - 1 >= 0 && grid[m - 1][n] == ‘1‘) { queue.offer((m - 1) * col + n); grid[m - 1][n] = ‘0‘; } if (m + 1 < row && grid[m + 1][n] == ‘1‘) { queue.offer((m + 1) * col + n); grid[m + 1][n] = ‘0‘; } if (n - 1 >= 0 && grid[m][n - 1] == ‘1‘) { queue.offer(m * col + n - 1); grid[m][n - 1] = ‘0‘; } if (n + 1 < col && grid[m][n + 1] == ‘1‘) { queue.offer(m * col + n + 1); grid[m][n + 1] = ‘0‘; } } } }
原文：https://www.cnblogs.com/xuanlu/p/11988532.html