200 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

Solution

class Solution {
public:
    void explore(vector<vector<char>>& grid, int i, int j ) {
        int m = grid.size();
        int n = grid[0].size();
        stack<pair<int, int>> s;
        s.push(pair<int, int>(i, j));
        while ( ! s.empty() ) {
            int n1 = s.top().first, n2 = s.top().second;
            s.pop();
            grid[n1][n2] = '0';
            if ( n1-1 >= 0 and grid[n1-1][n2] == '1' ) s.push(pair<int, int>(n1-1, n2));
            if ( n1+1 <= m-1 and grid[n1+1][n2] == '1' ) s.push(pair<int, int>(n1+1, n2));
            if ( n2-1 >= 0 and grid[n1][n2-1] == '1' ) s.push(pair<int, int>(n1, n2-1));
            if ( n2+1 <= n-1 and grid[n1][n2+1] == '1' ) s.push(pair<int, int>(n1, n2+1));
        }
        return;
    }
    int numIslands(vector<vector<char>>& grid) {
        int m = grid.size();
        if ( m == 0 ) return 0;
        int n = grid[0].size();
        if ( n == 0 ) return 0;
        int icount = 0;
        for ( int i = 0; i <= m-1; i++ ) {
            for ( int j = 0; j <= n-1; j++ ) {
                if ( grid[i][j] == '0' ) continue;
                icount += 1;
                explore(grid, i, j);
            }
        }
        return icount;
    }
};