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]
]
1
根据前一行生成下一行。
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public List<List<Integer>> generate(int numRows) {
List<List<Integer>> result = new ArrayList<List<Integer>>();
List<Integer> pre = new ArrayList<Integer>();
if(numRows <= 0) return result;
pre.add(1);
result.add(pre);
int a,b;
for(int i = 2; i <= numRows; i++){
List<Integer> eachLine = new ArrayList<Integer>();
for(int j = 1; j <= i; j++){
a = (j == 1) ? 0 : pre.get(j-2);
b = (j == i) ? 0 : pre.get(j-1);
eachLine.add(a+b);
}
result.add(eachLine);
pre = eachLine;
}
return result;
}
2 most votes
by rheaxu 方法很巧妙，row始终是当前row,其中的row.add(0,1)用的很好，java doc对于这个方法的解释是
void java.util.ArrayList.add(int index, Integer element) Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices). 每次会在list最前面添加1，后续元素右移。
}
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public class Solution {
public List<List<Integer>> generate(int numRows)
{
List<List<Integer>> allrows = new ArrayList<List<Integer>>();
ArrayList<Integer> row = new ArrayList<Integer>();
for(int i=0;i<numRows;i++)
{
row.add(0, 1);
for(int j=1;j<row.size()-1;j++)
row.set(j, row.get(j)+row.get(j+1));
allrows.add(new ArrayList<Integer>(row));
}
return allrows;