200. Number of Islands

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time: O(n x m)

space: O(1)

/**
 * @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[0].length; j++) {
            if (grid[i][j] !== '1') continue;
            traverse(grid, i, j);
            count++;
        }
    }

    return count;
};

function traverse(grid, row, col) {
    if (row < 0) return;
    if (row >= grid.length) return;
    if (col < 0) return;
    if (col >= grid[0].length) return;
    if (grid[row][col] !== '1') return;

    grid[row][col] = 2;

    const left = traverse(grid, row, col - 1);
    const right = traverse(grid, row, col + 1);
    const up = traverse(grid, row - 1, col);
    const down = traverse(grid, row + 1, col);

}