LeetCode – Pascal’s Triangle (Java)

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Given numRows, generate the first numRows of Pascal’s triangle. For example, given numRows = 5, the result should be:

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

Java Solution

public ArrayList<ArrayList<Integer>> generate(int numRows) {
	ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
	if (numRows <= 0)
		return result;
 
	ArrayList<Integer> pre = new ArrayList<Integer>();
	pre.add(1);
	result.add(pre);
 
	for (int i = 2; i <= numRows; i++) {
		ArrayList<Integer> cur = new ArrayList<Integer>();
 
		cur.add(1); //first
		for (int j = 0; j < pre.size() - 1; j++) {
			cur.add(pre.get(j) + pre.get(j + 1)); //middle
		}
		cur.add(1);//last
 
		result.add(cur);
		pre = cur;
	}
 
	return result;
}

3 thoughts on “LeetCode – Pascal’s Triangle (Java)”

  1. 1+2+3+4+…+n = n(n+1)/2 = ((n^2) + n) / 2
    Meaning O(n^2) time.
    Space is the same as you create a “memory unit” on each iteration unit.

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