119. Pascal's Triangle II

Description:

Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?

My code:

/**
 * @param {number} rowIndex
 * @return {number[]}
 */
var getRow = function(rowIndex) {
    //第 n 行的第  k 个数字为组合数 C (k - 1)/(n - 1)
    var getFactorial = function(num) { // 求阶乘
        let factorial = 1;
        for(let i = 1; i <= num; i++) {
            factorial *= i;
        }
        return factorial;
    };
    var arr = [];
    for(let i = 0; i <= rowIndex; i++) {
        if(i == 0 || i == rowIndex) {
            arr.push(1);
        } else {
            arr.push(getFactorial(rowIndex) / (getFactorial(i) * getFactorial(rowIndex - i)));
        }
    }
    return arr;
};

Note: 题目说有没有办法优化到空间复杂度为O(k)，暂时还没想到……