LeetCode: Pascal's Triangle II Solution

Approach

Build Pascal's Triangle of rowIndex + 1 rows

Get the last row

Implementation

1var getRow = function (rowIndex) {
2  const numRows = rowIndex + 1
3
4  const res = [[1]]
5
6  for (let rowIndex = 1; rowIndex < numRows; rowIndex++) {
7    const row = Array.from(
8      { length: rowIndex + 1 },
9      (_, elIndex) =>
10        (res[rowIndex - 1][elIndex] || 0) +
11        (res[rowIndex - 1][elIndex - 1] || 0)
12    )
13    res.push(row)
14  }
15
16  return res.pop()
17}

Implementation (O(rowIndex) space)

In-place replacement based on previous state

1i = 1: 1 0 0 0
2i = 2: 1 1 0 0
3i = 3: 1 2 1 0
4i = 4: 1 3 3 1

1var getRow = function (rowIndex) {
2  const numRows = rowIndex + 1
3  const res = Array(numRows).fill(0)
4  res[0] = 1
5
6  for (let i = 1; i < numRows; i++) {
7    for (let j = i; j >= 1; j--) {
8      res[j] += res[j - 1]
9    }
10  }
11
12  return res
13}