[LeetCode 题解]: 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]
]

产生杨辉三角，比较简单，注意几个边界条件：

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        vector<int> vi;
        vector<vector<int> > ans;
        int i,j;
        vi.push_back(1);
        ans.clear();   // 注意初始化
 9         if(numRows<=0) return ans;
10         if(1==numRows)
11         {
12             ans.push_back(vi);
13             return ans;
14         }
15         if(2==numRows)
16         {
17             ans.push_back(vi);
18             vi.push_back(1);
19             ans.push_back(vi);
20             return ans;
21         }

        ans.push_back(vi);
        vi.push_back(1);
        ans.push_back(vi);
        for(i=2;i<numRows;i++)
        {
            vi.clear();
            vi.push_back(1);
            for(j=1;j<i;j++)
            {
                vi.push_back(ans[i-1][j-1]+ans[i-1][j]);
            }
            vi.push_back(1);
            ans.push_back(vi);
        }
        return ans;
    }
};

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