LeetCode 118. 杨辉三角 Pascal's Triangle

 

 

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int> > res;
        for (int i = 1; i <= numRows; ++i)  //注意此时左右都是闭区间
        {
            vector<int> val(i);  //第i层共i个数
            val[0] = 1; val[i - 1] = 1;
            if (i <= 2)  //前两层
            {
                res.push_back(val);
                continue;
            }
            for (int j = 1; j < i - 1; ++j)
                val[j] = res[i - 2][j - 1] + res[i - 2][j];  //i起点为1,因此-2
            res.push_back(val);
        }
        return res;
    }
};