[leetcode]Climbing Stairs

问题：

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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?

分析：

   int climbStairs(int n) {
        if(n <= 2) return n;
        int f1 = 1;
        int f2 = 2;
        int f3;
        for(int i = 3; i <= n; ++i)
        {
            f3 = f2 + f1;
            f1 = f2;
            f2 = f3;
        }
        return f3;
    }