70. Climbing Stairs

You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

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

class Solution {
public:
    int climbStairs(int n) {
        if(n < 1)   return 0;
        int oneStep = 1;
        int twoStep= 1;
        int res = 1;

        for(int i = 2; i <= n; i++){
            res = oneStep + twoStep;
            twoStep = oneStep;
            oneStep = res;
        }
        return res;
    }
};