【LeetCode】Pascal's Triangle II （杨辉三角）

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?

注意：只开辟O(K)的内存空间，所以只保存前一组数列和当前数组，并不断滚动更新即可。

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<int> res;
        vector<int> pre;
        if(rowIndex == 0)
        {
            res.push_back(1);
            return res;
        }
        pre.push_back(1);
        for(int i = 1; i <= rowIndex; i++)
        {
            res.push_back(1);
            for(int j = 1; j < i+1; j++)
            {
                res.push_back( pre[j-1] + (j < pre.size() ? pre[j] : 0) );
            }
            pre.clear();
            for(int k = 0; k < res.size(); k++)
            {
                pre.push_back(res[k]);
            }
            res.clear();
        }
        return pre;
    }
};