【LeetCode】Pascal's Triangle

Pascal's Triangle 
Total Accepted: 13674 Total Submissions: 43141 My Submissions
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]
]

【解题思路】
1、numRows表示有几行，注意列表从0开始。
2、每一行的列表是这样构成的，开头和结尾都是1，中间是前一个列表相邻两个数之和。

Java AC 364ms

public class Solution {
    public ArrayList<ArrayList<Integer>> generate(int numRows) {
        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
        if(numRows <= 0){
            return list;
        }
        ArrayList<Integer> firstList = new ArrayList<Integer>();
        firstList.add(1);
        list.add(firstList);
        for(int i = 1; i < numRows; i++){
            ArrayList<Integer> secList = new ArrayList<Integer>();
            ArrayList<Integer> tempList = list.get(i-1);
            int size = tempList.size();
            secList.add(1);
            for(int j = 1; j < size; j++){
                secList.add(tempList.get(j-1) + tempList.get(j));
            }
            secList.add(1);
            list.add(secList);
        }
        return list;
    }
}

Python AC 120ms

class Solution:
    # @return a list of lists of integers
    def generate(self, numRows):
        list = []
        if numRows <= 0:
            return []
        list.append([1])
        for i in range(1, numRows):
            tempList = list[i-1]
            rowList = []
            rowList.append(1)
            for j in range(1,len(tempList)):
                rowList.append(tempList[j-1] + tempList[j])
            rowList.append(1)
            list.append(rowList)
        return list