【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;
}
};