[LeetCode] 200. Number of Islands

轉自LeetCode

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:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

<Solution>
典型的 DFS 題目，想法如下

• 找到入口點後，把所有連在一起的 '1' 都找出來，並換成其他字元

• 每找到一次入口點，答案就加 1

code 如下

C++

	class Solution {
	private:
	int row;
	int col;
	public:
	int numIslands(vector<vector<char>>& grid) {
	int ans = 0;
	if(!grid.empty()) {
	row = grid.size();
	col = grid[0].size();
	for(int r = 0; r < row; r++) {
	for(int c = 0; c < col; c++) {
	if(grid[r][c] == '1') {
	dfs(r, c, grid);
	++ans;
	}
	}
	}
	}
	return ans;
	}
	void dfs(const int &curR, const int &curC, vector<vector<char>> &grid) {
	grid[curR][curC] = 'x';
	for(int r = -1; r < 2; r += 2) {
	int nextR = curR + r;
	if((nextR >= 0 && nextR < row) &&
	grid[nextR][curC] == '1') {
	dfs(nextR, curC, grid);
	}
	}
	for(int c = -1; c < 2; c += 2) {
	int nextC = curC + c;
	if((nextC >= 0 && nextC < col) &&
	grid[curR][nextC] == '1') {
	dfs(curR, nextC, grid);
	}
	}
	}
	};

view raw numberOfIslands.cpp hosted with ❤ by GitHub

Java

	class Solution {
	public int numIslands(char[][] grid) {
	if(grid.length == 0 || grid[0].length == 0) {
	return 0;
	}
	final int row = grid.length;
	final int col = grid[0].length;
	int ans = 0;
	for(int r = 0; r < row; r++) {
	for(int c = 0; c < col; c++) {
	if(grid[r][c] == '1') {
	dfs(grid, r, c);
	++ans;
	}
	}
	}
	return ans;
	}
	private void dfs(char[][] grid, int row, int col) {
	if(row < 0 || row >= grid.length) {
	return;
	}
	else if(col < 0 || col >= grid[0].length) {
	return;
	}
	else if(grid[row][col] == 'x' || grid[row][col] == '0') {
	return;
	}
	grid[row][col] = 'x';
	for(int r = -1; r < 2; r += 2) {
	dfs(grid, row + r, col);
	}
	for(int c = -1; c < 2; c += 2) {
	dfs(grid, row, col + c);
	}
	}
	}

view raw numIslands.java hosted with ❤ by GitHub

kotlin

	class Solution {
	fun numIslands(grid: Array<CharArray>): Int {
	var ans = 0
	for(r in grid.indices) {
	for(c in grid[0].indices) {
	if(grid[r][c] == '1') {
	++ans
	dfs(grid, r, c)
	}
	}
	}
	return ans
	}
	fun dfs(grid: Array<CharArray>, row: Int, col: Int) {
	when {
	row < 0 || row > grid.lastIndex -> return
	col < 0 || col > grid[0].lastIndex -> return
	grid[row][col] == '0' || grid[row][col] == '*' -> return
	else -> {
	grid[row][col] = '*'
	for(r in -1..1 step 2) {
	dfs(grid, row+r, col)
	}
	for(c in -1..1 step 2) {
	dfs(grid, row, col+c)
	}
	}
	}
	}
	}

view raw number_of_islands.kt hosted with ❤ by GitHub