求Pascal三角的某一层，要求O(n)复杂度。

vector
     
       getRow(int rowIndex) {    vector
      
        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;}