class Solution {
 public:
  int climbStairs(int n) {
    if (n == 0 || n == 1) {
      return 1;
    }
    vector<int> dp(n+1);
    dp[1] = 1;
    dp[2] = 2;
    for (int i = 3; i <= n; i++) {
      dp[i] = dp[i - 1] + dp[i - 2];
    }
    return dp[n];
  }
};