岛屿数量 | Alvin's note

leetcode: 给你一个由 '1'（陆地）和 '0'（水）组成的的二维网格，请你计算网格中岛屿的数量。

岛屿总是被水包围，并且每座岛屿只能由水平方向和/或竖直方向上相邻的陆地连接形成。

此外，你可以假设该网格的四条边均被水包围。

输入：grid = [
  ["1","1","1","1","0"],
  ["1","1","0","1","0"],
  ["1","1","0","0","0"],
  ["0","0","0","0","0"]
]
输出：1

输入：grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
输出：3

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

  function toZero(i, j) {
    if (i < 0 || i >= m || j < 0 || j >= n || grid[i][j] === '0') return;
    grid[i][j] = '0';
    toZero(i + 1, j);
    toZero(i - 1, j);
    toZero(i, j + 1);
    toZero(i, j - 1);
  }

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

  return count;
};