重构多层for循环-leetcode的118. Pascal's Triangle启发

原因：多位数组遍历会产生多层循环，循环中再嵌套逻辑会使得函数变得庞大，难以维护。

解决方法：内层的for循环包装成函数。如下：

118. Pascal's Triangle

Given a non-negative integer numRows, generate the first numRows of Pascal's triangle.

In Pascal's triangle, each number is the sum of the two numbers directly above it.

Example:

Input: 5
Output:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

func generate(numRows int) [][]int {
    res := [][]int{}
    if numRows == 0 {
        return res
    }
    res = append(res,[]int{1})
    for i:=1; i<numRows; i++ {
        res = append(res,genNext(res[i-1]))
    }
    return res
}

func genNext(p []int) []int {
    r := make([]int,1,len(p)+1)
    r = append(r,p...)
    for i:=0; i<len(r)-1; i++ {
        r[i] = r[i] + r[i+1]
    }
    return r
}