Pascal's Triangle II leetcode

Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?

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依次在上一行的基础上计算下一行，技巧就是从后向前计算，避免前面的计算影响后面的计算

vector<int> getRow(int rowIndex) {
    vector<int> ret(rowIndex+, );
    ret[] = ;
    for (int i = ; i <= rowIndex; ++i)
        for (int j = i; j >= ; --j)
            ret[j] += ret[j - ];
    return ret;
}