509. 斐波那契数

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impl Solution {
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    pub fn fib(n: i32) -> i32 {
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        if n == 0 {
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            return 0;
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        } else if n == 1 {
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            return 1;
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        }
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        let mut data = vec![0, 1];
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        let mut next = 0;
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        let mut i = 1;
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        while i < n {
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            data[next] = data[0] + data[1];
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            i += 1;
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            if i == n {
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                return data[next];
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            }
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            if next == 0 {
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                next = 1;
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            } else {
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                next = 0;
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            }
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        }
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        data[next]
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    }
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}