Leetcode•Jul 18, 2025

Number of Islands

Hazrat Ali

Leetcode

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

Solution
/**
 * @param {character[][]} grid
 * @return {number}
 */
const numIslands = grid => {
  if (!grid || grid.length === 0) {
    return 0;
  }

  const m = grid.length;
  const n = grid[0].length;

  let count = 0;

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

  return count;
};

const dfs = (grid, m, n, i, j) => {
  if (i < 0 || i >= m || j < 0 || j >= n || grid[i][j] === '0') {
    return;
  }

  grid[i][j] = '0';

  const dx = [0, 0, -1, 1];
  const dy = [-1, 1, 0, 0];

  for (let k = 0; k < 4; k++) {
    dfs(grid, m, n, i + dx[k], j + dy[k]);
  }
};

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