Pascal's Triangle

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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]]

易错点： pre 的时候要判断 i > 0

public class Solution {    public List<List<Integer>> generate(int numRows) {        List<List<Integer>> ret = new ArrayList<List<Integer>>();        if(numRows < 1)            return ret;        List<Integer> first = new ArrayList<Integer>();        first.add(1);        ret.add(first);        for(int i = 1; i < numRows; i++){            List<Integer> cur = new ArrayList<Integer>();            List<Integer> pre = ret.get(i - 1);            for(int j = 0; j <=  i; j++){                if(j == 0 || j == i){                    cur.add(1);                    continue;                }                cur.add(pre.get(j - 1) + pre.get(j));            }            ret.add(cur);        }        return ret;    }}

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