Number of Islands

07/16/2016 Depth First Search Breadth First Search Union Find

Question
Solution

Question

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:

Answer: 1

Example 2:

Answer: 3

Solution

Result: Accepted Time: 116 ms

Here should be some explanations.

bool vis[4096][4096]={0};

int dfs(char ** grid,int x,int y,const int r,const int c)
{
    if(x < 0 || y < 0 || x >= r || y >= c)
        return 0;
    if(grid[x][y] == '0' || vis[x][y])
    return 0;
    vis[x][y] = 1;
    dfs(grid,x+1,y,r,c);
    dfs(grid,x-1,y,r,c);
    dfs(grid,x,y-1,r,c);
    dfs(grid,x,y+1,r,c);

    return 1;
}

int numIslands(char** grid, int gridRowSize, int gridColSize) {
    memset(vis,0,sizeof(vis));
    int ret = 0;
    for(int x = 0; x < gridRowSize; x ++)
    for(int y = 0; y < gridColSize; y ++)
        ret += dfs(grid,x,y,gridRowSize,gridColSize);
    return ret;
}

Complexity Analytics

Time Complexity: $O(n)$
Space Complexity: $O(1)$