118. Pascal's Triangle (E)

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]
]

题意

生成前n行帕斯卡(杨辉)三角形。

思路

直接模拟操作生成每一行。

代码实现

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> ans = new ArrayList<>();
        if (numRows == 0) {
            return ans;
        }
        ans.add(new ArrayList<>());
        ans.get(0).add(1);
        for (int i = 1; i < numRows; i++) {
            ans.add(new ArrayList<>(ans.get(i - 1)));
            List<Integer> curRow = ans.get(i);
            for (int j = i; j >= 1; j--) {
                if (j == i) {
                    curRow.add(1);
                } else {
                    curRow.set(j, curRow.get(j) + curRow.get(j - 1));
                }
            }
        }
        return ans;
    }
}