## Pascal's Triangle

### 描述

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]
]


### 从左到右

// Pascal's Triangle
// 时间复杂度O(n^2)，空间复杂度O(n)
public class Solution {
public List<List<Integer>> generate(int numRows) {
List<List<Integer>> result = new ArrayList<>();
if(numRows == 0) return result;

for(int i = 2; i <= numRows; ++i) {
Integer[] current = new Integer[i]; // 本行
Arrays.fill(current, 1);
List<Integer> prev = result.get(i - 2); // 上一行

for(int j = 1; j < i - 1; ++j) {
current[j] = prev.get(j-1) + prev.get(j); // 左上角和右上角之和
}
}
return result;
}
}


### 从右到左

// Pascal's Triangle
// 时间复杂度O(n^2)，空间复杂度O(n)
public class Solution {
public List<List<Integer>> generate(int numRows) {
List<List<Integer>> result = new ArrayList<>();
List<Integer> array = new ArrayList<>();
for (int i = 1; i <= numRows; i++) {
for (int j = i - 2; j > 0; j--) {
array.set(j, array.get(j - 1) + array.get(j));
}