Pascal's Triangle - algorithm works in C++ but got 'Time Limit Exceeded' in java

Question

A leetcode problem: Given numRows, generate the first numRows of Pascal's triangle.

The C++ version of this algorithm got accepted by Leetcode. Could anyone tell me why this Java version cannot get accepted?

    public class Solution {

    public List<List<Integer>> generate(int numRows) {

    List<List<Integer>> result = new ArrayList<List<Integer>>();
    if (numRows == 0) return result;

    List<Integer> raw = new ArrayList<Integer>();
    raw.add(1);
    result.add(raw);
    if (numRows == 1) return result;

    List<Integer> row  = raw;
    List<Integer> row2 = raw;
    for (int i = 2; i <= numRows; i++) {
        for (int j = 0; j < row.size()-1; j++)
            row2.add(row.get(j) + row.get(j+1));
        row2.add(1);
        result.add(row2);
        row = row2;
        row2 = raw;
    }

    return result;
}
}

Answer 1

Could anyone tell me why this Java version cannot get accepted?

Because your code runs forever.

You have exactly one List<Integer> and it keeps growing:

for (int j = 0; j < row.size()-1; j++)
        row2.add(row.get(j) + row.get(j+1));

In each iteration you move one element forward and add one element, to the same list, essentially eternally chasing the end that keeps getting away.

(I had a friend at uni who initially also didn't understand how references work in Java and made a similar mistake with Lists. He got like a bazillion buttons in a Swing GUI.)