# 118.Pascals Triangle

**118.Pascals Traingle**

难度:Easy

> 给定一个非负整数 numRows，生成杨辉三角的前 numRows 行。 ![](https://upload.wikimedia.org/wikipedia/commons/0/0d/PascalTriangleAnimated2.gif) 在杨辉三角中，每个数是它左上方和右上方的数的和。

示例:

```
输入: 5
输出:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]
```

代码如下：

```
class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        
        if(numRows<=tri.size()) 
        {
            vector<vector<int>> res;
            for(int i=0;i<numRows;i++)
                res.push_back(tri[i]);
            return res;
        }
        int n=tri.size();
        while(n<numRows)
        {
            vector<int> tmp;
            tmp.push_back(tri[n-1][0]);
            for(int i=1;i<n;i++)
                tmp.push_back(tri[n-1][i]+tri[n-1][i-1]);
            tmp.push_back(tri[n-1][n-1]);
            tri.push_back(tmp);
            n++;
        }
        return tri;
    }
private:
    vector<vector<int>>tri={ {1},{1,1} };
};
```


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