杨辉三角

输入: 5
输出:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

const generate = function(numRows) {
    if (numRows === 0) return []

    const res = [[1]]

    if (numRows === 1) return res

    while (res.length < numRows) {
        const currentLine = [1]
        const lastLine = res[res.length - 1]
        const len = lastLine.length

        for (let i = 0; i < len; i++) {
            if ((i+1) === len) {
                currentLine.push(1)
            }else{
                currentLine.push(lastLine[i] + lastLine[i + 1])
            }
        }
        res.push(currentLine)
    }

    return res
};