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输入: numRows = 5
输出: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]

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输入: numRows = 1
输出: [[1]]

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impl Solution {
    /// ## 解题思路
    /// * 循环通过
    pub fn generate(num_rows: i32) -> Vec<Vec<i32>> {
        let mut res = vec![vec![1_i32]];
        for i in 1..num_rows {
            let mut row = res.last()
                            .unwrap()
                            .windows(2)
                            .map(|x| x[0] + x[1])
                            .collect();
            res.push([&[1], row.as_slice(), &[1]].concat());
        }
        res
    }
}