118-pascal's-triangle.js

/**
 * @param {number} numRows
 * @return {number[][]}
 */
const generate = function(numRows) {
  // row 0 => [1] length 0
  // row 1 => [1, 1] length 1
  // row 2 => [1, 2, 1] length 2
  // row 3 => [1, 3, 3, 1] length 3

  // current[i] = prev[i - 1] + prev[i]

  const res = [];
  for (let row = 0; row < numRows; row += 1) {
    if (row === 0) {
      res.push([1]);
      continue;
    }

    if (row === 1) {
      res.push([1, 1]);
      continue;
    }

    const newRow = [];
    const maxIdx = row;
    for (let i = 0; i <= maxIdx; i += 1) {
      if (i === 0 || i === maxIdx) {
        newRow.push(1);
      } else {
        newRow.push(res[row - 1][i - 1] + res[row - 1][i]);
      }
    }
    res.push(newRow);
  }

  return res;
};