118.杨辉三角

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