Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?

分析

返回第K行杨辉三角。使用O（k）空间，只需要从后向前依次计算即可。

/**
 * Return an array of size *returnSize.
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* getRow(int rowIndex, int* returnSize) {
    int *ans=(int*)malloc(sizeof(int)*(rowIndex+1));
    *returnSize=rowIndex+1;
    ans[0]=1;
    for(int i=1;i<=rowIndex;i++)
    {
        ans[i]=1;
        for(int j=i-1;j>=1;j--)
        {
            ans[j]=ans[j]+ans[j-1];
        }
    }
    return ans;
}