text: class Solution {
  public:
      int climbStairs(int n) {
          int dp0 = 1, dp1 = 1;
          for (int i = 2; i <= n; ++i) {
              int dp2 = dp0 + dp1;
              dp0 = dp1;
              dp1 = dp2;
          }
          return dp1;
      }
  };
  