class Solution {public:    int dp[1000] = {0};    int climbStairs(int n) {        if (dp[n] > 0) return dp[n];        if (n == 1) return dp[1] = 1;        else if (n == 2) return dp[2] = 2;        else return dp[n] = climbStairs(n - 1) + climbStairs(n - 2);    }};