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]
]

//Time: (m * n), Space: O(m * n)  
  public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        
        if (numRows <= 0) {
            return result;
        }
        
        List<Integer> first = new ArrayList<Integer>();
        first.add(1);
        result.add(first);//先把第一层特殊的放入result
        
        for (int i = 1; i < numRows; i++) {//注意i从1开始i因为在外面已经有了第0层
            List<Integer> temp = new ArrayList<Integer>();
            temp.add(1);//每层第一个
            
            for (int j = 1; j < i; j++) {////注意j从1开始i因为在外面已经有了第0列
                temp.add(result.get(i - 1).get(j) + result.get(i - 1).get(j - 1));
            }
            
            temp.add(1);//每层最后一个
            result.add(temp);
        }
        
        return result;
    }