LeetCode in Kotlin

119. Pascal’s Triangle II

Easy

Given an integer rowIndex, return the rowIndexth (0-indexed) row of the Pascal’s triangle.

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

Example 1:

Input: rowIndex = 3

Output: [1,3,3,1]

Example 2:

Input: rowIndex = 0

Output: [1]

Example 3:

Input: rowIndex = 1

Output: [1,1]

Constraints:

• 0 <= rowIndex <= 33

Follow up: Could you optimize your algorithm to use only O(rowIndex) extra space?

Solution

class Solution {
    fun getRow(rowIndex: Int): List<Int> {
        val buffer = IntArray(rowIndex + 1)
        buffer[0] = 1
        computeRow(buffer, 1)
        // Copy buffer to List of Integer.
        val ans: MutableList<Int> = ArrayList(buffer.size)
        for (j in buffer) {
            ans.add(j)
        }
        return ans
    }

    private fun computeRow(buffer: IntArray, k: Int) {
        if (k >= buffer.size) {
            return
        }
        var previous = buffer[0]
        for (i in 1 until k) {
            val tmp = previous + buffer[i]
            previous = buffer[i]
            buffer[i] = tmp
        }
        buffer[k] = 1
        computeRow(buffer, k + 1)
    }
}

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