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class Solution {
public:
    int climbStairs(int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int a1 = 1;
        int a2 = 2;
        if(n == 1)
        {
            return a1;
        }
        if(n == 2)
        {
            return a2;
        }
        int index = 3;
        int before = a2;
        int now = a1 + a2;
        while(true)
        {
            if(index == n)
            {
                return now;
            }
            int tmp = before;
            before = now;
            now = tmp + before;
            index++;
        }
    }
};