70. Climbing Stairs - cocoder39/coco39_LC GitHub Wiki

int climbStairs(int n) {
        if (n <= 0) {
            return 0;
        }
        
        vector<int> dp(n + 1); //dp[i] is ways of climbing to step i
        dp[1] = 1;
        dp[2] = 2;
        for (int i = 3; i <= n; i++) {
            dp[i] = dp[i - 2] + dp[i - 1];
        }
        return dp[n];
    }

int climbStairs(int n) {
        if (n <= 0) {
            return 0;
        }
        if (n == 1 || n == 2) {
            return n;
        }
        
        int twoStepBefore = 1; //n == 1
        int oneStepBefore = 2; //n == 2
        int res;
        for (int i = 3; i <= n; i++) {
            res = twoStepBefore + oneStepBefore;
            
            twoStepBefore = oneStepBefore;
            oneStepBefore = res;
        }
        return res;
    }