70. Climbing Stairs

Solution - DP

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

• T:
  $O(N)$
• S:
  $O(N)$

Solution - DP improved

class Solution {
public:
    int climbStairs(int n)
    {
        int first = 1, second = 1;
        for(int i = 2; i <= n; i++)
        {
            int temp = first;
            first = first + second;
            second = temp;
        }
        return first;
    }
};

• T:
  $O(N)$
• S:
  $O(1)$