118. Pascal's Triangle 杨辉三角

118. 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]]

按照规律写两个循环即可，但是要注意新值覆盖旧值的问题，而且只能计算一半，剩下一半对称复制。

class Solution {public:    vector<vector<int>> generate(int numRows) {        vector<int> happy;        vector<vector<int>> res;        if(numRows==0) return res;        happy.push_back(1);        res.push_back(happy);        if(numRows==1) return res;        happy.push_back(1);        res.push_back(happy);        if(numRows==2) return res;        for(int i=3;i<=numRows;i++)        {            for(int j=(happy.size())/2;j>=1;j--)//这个循环要从大到小            {                happy[j]=happy[j]+happy[j-1];               happy[happy.size()-j]=happy[j];            }            happy.push_back(1);            res.push_back(happy);        }        return res;    }};