118. Pascal's Triangle 打印杨辉三角

难度：【easy】

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