Pascal's Triangle

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]
]

New version!

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        vector<vector<int> > result;
        if(numRows < 1) return result;
        
        for(int i = 0; i < numRows; i++){
            vector<int> temp;
            for(int j = 0; j <= i; j++){
                if(j == 0 || j == i)
                    temp.push_back(1);
                else
                    temp.push_back(result[i - 1][j - 1] + result[i - 1][j]);
            }
            result.push_back(temp);
        }
        return result;
    }
};

Analyse: Considering numRows =0 , 1, 2 and other circumustance. 

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        vector<vector<int> > result;
        if(numRows == 0) return result;
       
        vector<int> row0;
        row0.push_back(1);
        result.push_back(row0);
        if(numRows == 1) return result;
        
        row0.push_back(1);
        result.push_back(row0);
        if(numRows == 2) return result;
        
        for(int i = 3; i <= numRows; i++){
            vector<int> row;
            vector<int> previous = result[result.size()-1];
            row.push_back(1);
            for(int j = 1; j < i - 1; j++){
                row.push_back(previous[j-1] + previous[j]);
            }
            row.push_back(1);
            result.push_back(row);
        }
        return result;
    }
};