70. Climbing Stairs

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