LeetCode-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]
]

---

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<vector<int> > ret;
        if(numRows<=0)return ret;
        ret.resize(1);
        ret[0].resize(1);
        ret[0][0]=1;
        for(int i=1;i<numRows;i++){
            vector<int> layer;
            layer.resize(i+1);
            layer[0]=ret[i-1][0];
            layer[i]=ret[i-1][i-1];
            for(int j=1;j<i;j++){
                layer[j]=ret[i-1][j-1]+ret[i-1][j];
            }
            ret.push_back(layer);
        }
        return ret;
    }
};