【leetcode】Pascal's Triangle II

Question：

Given an index k, return the k^th 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?

Anwser 1：   

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> ret;
        
        for(int i = 0; i <= rowIndex; i++){
            if(i == 0) {
                ret.push_back(1);
                continue;
            }            
            
            for(int j = i; j >= 0; j--){    // from end to begin
                if(j == 0) {
                    ret[0] = 1;
                } else if(j == i){
                    ret.push_back(1);
                }else {
                    ret[j] = ret[j-1] + ret[j];
                }
            }
        }
        
        return ret;
    }
};