LeetCode: 119. Pascal 解题笔记

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119. Pascal’s Triangle II

LeetCode - Pascal’s Triangle II

Given a non-negative index k where k ≤ 33, return the k^th index row of the Pascal’s triangle.

Note that the row index starts from 0.

In Pascal’s triangle, each number is the sum of the two numbers directly above it.

Example:

Input: 3
Output: [1,3,3,1]

Follow up:

Could you optimize your algorithm to use only O(k) extra space?

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/**
 * Runtime: 1 ms, faster than 89.02% of Java online submissions for Pascal's Triangle II.
 *
 * Memory Usage: 33.7 MB, less than 6.17% of Java online submissions for Pascal's Triangle II.
 */
public List<Integer> getRow(int rowIndex) {
    List<Integer> result = new ArrayList<>(rowIndex * 3 / 2 + 1);
    result.add(1);
    for (int i = 1; i <= rowIndex; i++) {
        int left = 1;
        for (int j = 1; j < i + 1; j++) {
            int right = j == result.size() ? 0 : result.get(j);
            int element = left + right;
            if (j == result.size()) {
                result.add(element);
            } else {
                result.set(j, element);
            }
            left = right;
        }
    }
    return result;
}