Pascal's Triangle II 解答

Question

Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Solution

Similar with Pascal's Triangle. Note that the index starts from 0.

public class Solution {
    public List<Integer> getRow(int rowIndex) {
        List<Integer> result = new ArrayList<Integer>();
        List<Integer> prev = new ArrayList<Integer>();
        if (rowIndex < 0)
            return result;
        result.add(1);
        prev.add(1);
        while (rowIndex > 0) {
            result = new ArrayList<Integer>();
            result.add(1);
            int length = prev.size();
            for (int i = 0; i < length - 1; i++)
                result.add(prev.get(i) + prev.get(i + 1));
            result.add(1);
            prev = result;
            rowIndex--;
        }
        return result;
    }
}

We can also only use one list.

public List<Integer> getRow(int rowIndex) {
    ArrayList<Integer> result = new ArrayList<Integer>();
 
    if (rowIndex < 0)
        return result;
 
    result.add(1);
    for (int i = 1; i <= rowIndex; i++) {
        for (int j = result.size() - 2; j >= 0; j--) {
            result.set(j + 1, result.get(j) + result.get(j + 1));
        }
        result.add(1);
    }
    return result;
}