70. 爬楼梯

/**
 * @param {number} n
 * @return {number}
 */
var climbStairs = function (n) {
  const dp = new Array(n + 1).fill(0)
  dp[0] = 1
  dp[1] = 1
  for (let i = 2; i < dp.length; i++) {
    dp[i] = dp[i - 2] + dp[i - 1]
  }
  return dp[n]
}