70.爬楼梯.js

/*
 * @lc app=leetcode.cn id=70 lang=javascript
 *
 * [70] 爬楼梯
 */

// @lc code=start
/**
 * @param {number} n
 * @return {number}
 */
// dp 与斐波那契数列解法一样 f(n) = f(n-1) + f(n-2)
var climbStairs = function(n) {
  if (n <= 2) {
    return n;
  }

  let f1 = 1,
    f2 = 2,
    f3 = null;

  for (let i = 3; i <= n; i++) {
    f3 = f2 + f1;
    f1 = f2;
    f2 = f3;
  }

  return f2;
};
// @lc code=end