解析

//!  n,1->for循环n次，常数空间
class Solution {
 public:
  int climbStairs(int n) {
    int a = 0, b = 1, c = 0;
    for (int i = 1; i <= n; ++i) {
      c = a + b;
      a = b;
      b = c;
    }

    return c;
  }
};