119. Pascal's Triangle II

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?

还是杨辉三角，注意v[i - 1] + v[i]  递推要逆序。哈

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        rowIndex ++;
        vector<int>v(rowIndex );
        if (rowIndex == 1) v[0] = 1;
        else if (rowIndex == 2) v[0] = 1,v[1] = 1;
        else {
            v[0] = 1,v[1] = 1;
            for (int i = 2; i < rowIndex; ++i) {
                v[i] = v[i - 1];
                for (int j = i; j >= 0; --j) {
                    if (j == 0) v[i] = v[0];
                    else v[j] = v[j] + v[j - 1];
                }
            }
        }
        return v;
    }
};