getRow(rowIndex) {
        Util.need(rowIndex >= 0 && rowIndex <= this.height(), "rowIndex >= 0 && rowIndex <= this.height()");
        let row = [];
        for (let c = 0; c < this._width; c++) {
            row.push(this.cell(c, rowIndex));
        }
        return row;
    }
@param {!int} rowIndex
@returns {!Array.<!Complex>}