【leetcode】Pascal's Triangle

Question：      

Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5,
Return

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

Anwser 1：      

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector< vector<int> > ret;
        
        for(int i = 0; i < numRows; i++){
            vector<int> tmp;
            tmp.clear();
            if(i == 0) {
                tmp.push_back(1);
                ret.push_back(tmp);
                continue;
            }
            
            for(int j = 0; j <= i; j++){
                if(j == 0) {
                    tmp.push_back(1);
                } else if(i == j){
                    tmp.push_back(1);
                } else {
                    tmp.push_back( ret[i-1][j-1] + ret[i-1][j] );   // last row
                }
            }
            ret.push_back(tmp);
        }
        
        return ret;
    }
};