leetcode 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?

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class Solution {
public:
vector<int> getRow(int rowIndex) {
vector<int> vec;
if(rowIndex < 0) return vec;
int prev = 0;

for(int i=0; i <= rowIndex; i++) {
if(i==0)
vec.push_back(1);

for(int j=vec.size()-1; j > 0; j--) {
vec[j] = vec[j]+vec[j-1];
}

if(i > 0)
vec.push_back(1);
}

return vec;
}
};