leetcode 119 Pascal's Triangle II

求Pascal三角的某一层，要求O(n)复杂度。

vector<int> getRow(int rowIndex) {
    vector<int> ret(++rowIndex);
    
    for (int i = 0; i < rowIndex; ++i) {
        int last;
        for (int j = 0; j <= i; ++j) {
            if (j == 0 || j == i) {
                ret[j] = 1;
                last = 1;
            }
            else {
                int temp = ret[j];
                ret[j] = last + ret[j];
                last = temp;
            }
        }
    }
    return ret;
}