螺旋矩阵

var spiralOrder = function(matrix) {
    if (!matrix.length || !matrix[0].length) {
        return [];
    }
    const rows = matrix.length, columns = matrix[0].length;
    const visited = new Array(rows).fill(0).map(() => new Array(columns).fill(false));
    const total = rows * columns;
    const order = new Array(total).fill(0);
    let directionIndex = 0, row = 0, column = 0;
    const directions = [[0, 1], [1, 0], [0, -1], [-1, 0]];
    for (let i = 0; i < total; i++) { 
        order[i] = matrix[row][column];
        visited[row][column] = true;
        const nextRow = row + directions[directionIndex][0], nextColumn = column + directions[directionIndex][1];
        if (!(0 <= nextRow && nextRow < rows && 0 <= nextColumn && nextColumn < columns && !(visited[nextRow][nextColumn]))) {
            directionIndex = (directionIndex + 1) % 4;
        }
        row += directions[directionIndex][0];
        column += directions[directionIndex][1];
    }
    return order;
};