LeetCode Q200: Number of Islands

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

	class Solution {
	public:
	int numIslands(vector<vector<char>>& grid) {
	int res=0;
	if(grid.empty())
	return res;
	vector<vector<int> > accessed(grid.size(), vector<int>(grid[0].size(), 0));
	for(int i=0; i<grid.size(); i++){
	for(int j=0; j<grid[i].size(); j++){
	if(grid[i][j]=='0' || accessed[i][j]==1)
	continue;
	queue<pair<int, int> > Q;
	Q.push(pair<int, int> (i, j));
	accessed[i][j]=1;
	while(!Q.empty()){
	pair<int, int> p=Q.front();
	Q.pop();
	if(p.first>0&&grid[p.first-1][p.second]=='1'&&accessed[p.first-1][p.second]==0){
	Q.push(pair<int, int> (p.first-1, p.second));
	accessed[p.first-1][p.second]=1;
	}
	if(p.first<grid.size()-1&&grid[p.first+1][p.second]=='1'&&accessed[p.first+1][p.second]==0){
	Q.push(pair<int, int> (p.first+1, p.second));
	accessed[p.first+1][p.second]=1;
	}
	if(p.second>0&&grid[p.first][p.second-1]=='1'&&accessed[p.first][p.second-1]==0){
	Q.push(pair<int, int> (p.first, p.second-1));
	accessed[p.first][p.second-1]=1;
	}
	if(p.second<grid[0].size()-1&&grid[p.first][p.second+1]=='1'&&accessed[p.first][p.second+1]==0){
	Q.push(pair<int, int> (p.first, p.second+1));
	accessed[p.first][p.second+1]=1;
	}
	}
	res++;
	}
	}
	return res;
	}
	};

view raw Q200NumberOfIslands.cpp hosted with ❤ by GitHub

Round 2 solution:
DFS

	class Solution {
	public:
	void helper(vector<vector<char> >& grid, vector<vector<bool> >& mark, int r, int c){
	if(r<0||r>=grid.size()||c<0||c>=grid[0].size()||grid[r][c]=='0'||mark[r][c]==true)
	return;
	mark[r][c]=true;
	int steps[] = {0, 0, 0, -1, 0, 1, -1, 0, 1, 0};
	for(int i=0; i<5; i++)
	helper(grid, mark, r+steps[i*2], c+steps[i*2+1]);
	}
	int numIslands(vector<vector<char>>& grid) {
	int res = 0;
	if(grid.empty())
	return res;
	vector<vector<bool> > mark(grid.size(), vector<bool>(grid[0].size(), false));
	for(int i=0; i<grid.size(); i++){
	for(int j=0; j<grid[0].size(); j++){
	if(grid[i][j]=='1'&&mark[i][j]==false){
	res++;
	helper(grid, mark, i, j);
	}
	}
	}
	return res;
	}
	};

view raw Q200Rnd2.cpp hosted with ❤ by GitHub

Rnd3 sol:

	class Solution {
	public:
	void helper(vector<vector<char>>& grid, int row, int col){
	if(row<0||col<0||row>=grid.size()||col>=grid[0].size()||grid[row][col]=='0')
	return;
	grid[row][col]='0';
	helper(grid, row, col-1);
	helper(grid, row, col+1);
	helper(grid, row-1, col);
	helper(grid, row+1, col);
	}
	int numIslands(vector<vector<char>>& grid) {
	int res = 0;
	if(grid.empty())
	return res;
	for(int i=0; i<grid.size(); i++){
	for(int j=0; j<grid[0].size(); j++){
	if(grid[i][j]=='0')
	continue;
	helper(grid, i, j);
	res++;
	}
	}
	return res;
	}
	};

view raw Q200Rnd3.cpp hosted with ❤ by GitHub