leetcode: 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (n == 0)
            return 0;
        if (n == 1)
            return 1;
        if (n == 2)
            return 2;
                     
        int curStep;
        int preStep     = 2;
        int preTwoStep  = 1;
            
        for (int i=3; i<=n; i++)
        {
            curStep    = preStep + preTwoStep;
            preTwoStep = preStep;
            preStep    = curStep;
        }
             
        return curStep;
    }
};