200. Number of Islands

Description

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.

 

Example 1:

Input: grid = [
  ["1","1","1","1","0"],
  ["1","1","0","1","0"],
  ["1","1","0","0","0"],
  ["0","0","0","0","0"]
]
Output: 1

Example 2:

Input: grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
Output: 3

 

Constraints:

• m == grid.length
• n == grid[i].length
• 1 <= m, n <= 300
• grid[i][j] is '0' or '1'.

 

Solutions

Solution: Depth-First Search

• Time complexity: O(mn)
• Space complexity: O(mn)

 

JavaScript

/**
 * @param {character[][]} grid
 * @return {number}
 */
const numIslands = function (grid) {
  const m = grid.length;
  const n = grid[0].length;
  const findIsland = (row, col) => {
    if (row < 0 || col < 0 || row >= m || col >= n) return;
    if (grid[row][col] !== '1') return;
    grid[row][col] = '#';
    findIsland(row - 1, col);
    findIsland(row + 1, col);
    findIsland(row, col - 1);
    findIsland(row, col + 1);
  };
  let result = 0;

  for (let row = 0; row < m; row++) {
    for (let col = 0; col < n; col++) {
      if (grid[row][col] !== '1') continue;
      findIsland(row, col);
      result += 1;
    }
  }
  return result;
};