LeetCode | Pascal's Triangle

题目

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]
]

逐层构造，注意边界条件。

代码

import java.util.ArrayList;

public class PascalTriangle {
	public ArrayList
  
   > generate(int numRows) {
		ArrayList
   
    > results = new ArrayList
    
     >(); if (numRows <= 0) { return results; } ArrayList
     
       firstRow = new ArrayList
      
       (); firstRow.add(1); results.add(firstRow); ArrayList
       
         lastRow = firstRow; for (int i = 1; i < numRows; ++i) { ArrayList
        
          row = new ArrayList
         
          (); row.add(1); for (int j = 1; j < i - 1; ++j) { row.add(lastRow.get(j - 1) + lastRow.get(j)); } row.add(1); results.add(row); lastRow = row; } return results; } }