Climbing Stairs LeetCode solution in C++

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class Solution {
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 public:
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  int climbStairs(int n) {
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    if (n <= 2) return n;
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    int prev = 2, prev2 = 1, res;
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    for (int i = 3; i <= n; i++) {
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      res = prev + prev2;
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      prev2 = prev;
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      prev = res;
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    }
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    return res;
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  }
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};