Given a non-negative index k where k ≤ 33, return the k-th index row of the Pascal’s triangle.

Note that the row index starts from 0.

In Pascal’s triangle, each number is the sum of the two numbers directly above it.

Example:

Input: 3
Output: [1,3,3,1]

Follow up:

Could you optimize your algorithm to use only O(k) extra space?

solution

Time complexity : O(k)
Space complexity : O(k)

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

例子：