0200-number-of-islands.js

/**
 * 200. Number of Islands
 * https://leetcode.com/problems/number-of-islands/
 * Difficulty: Medium
 *
 * Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water),
 * return 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.
 */

/**
 * @param {character[][]} grid
 * @return {number}
 */
var numIslands = function(grid) {
  let count = 0;

  for (let i = 0; i < grid.length; i++) {
    for (let j = 0; j < grid[i].length; j++) {
      if (grid[i][j] === '1') {
        count++;
        dfs(i, j);
      }
    }
  }

  function dfs(i, j) {
    if (i < 0 || i >= grid.length || j < 0 || j >= grid[i].length || grid[i][j] === '0') {
      return;
    }

    grid[i][j] = '0';

    dfs(i + 1, j);
    dfs(i - 1, j);
    dfs(i, j + 1);
    dfs(i, j - 1);
  }

  return count;
};