LeetCode - Pascal's Triangle

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Given an integer numRows, return the first numRows of Pascal's triangle.

In Pascal's triangle, each number is the sum of the two numbers directly above it as shown:

 

Example 1:

Input: numRows = 5
Output: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]

Example 2:

Input: numRows = 1
Output: [[1]]

Constraints:

• 1 <= numRows <= 30

풀이(Kotlin):

fun generate(numRows: Int): List<List<Int>> {
    val ans = mutableListOf<List<Int>>(listOf(1))
    if (numRows == 1) return ans
    ans.add(listOf(1, 1))
    if (numRows == 2) return ans
    for (i in 2 until  numRows) {
        val before = ans[i-1]
        val new = arrayListOf<Int>(1, 1)
        for (j in 0 until before.lastIndex) {
            new.add(j+1, before[j] + before[j+1])
        }
        ans.add(new)
    }
    return  ans
}