LeetCode: 118. Pascal 解题笔记

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118. Pascal’s Triangle

LeetCode - Pascal’s Triangle

Given a non-negative integer numRows, generate the first numRows of Pascal’s triangle.

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

Example:

Input: 5
Output:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

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/**
 * Runtime: 0 ms, faster than 100.00% of Java online submissions for Pascal's Triangle.
 *
 * Memory Usage: 34 MB, less than 7.23% of Java online submissions for Pascal's Triangle.
 */
public List<List<Integer>> generate(int numRows) {
    if (numRows < 1) {
        return Collections.emptyList();
    }
    List<List<Integer>> result = new ArrayList<>(numRows);
    result.add(Arrays.asList(1));
    for (int i = 1; i < numRows; i++) {
        List<Integer> parent = result.get(i - 1);
        int length = parent.size() + 1;
        List<Integer> current = new ArrayList<>(length);
        current.add(1);
        for (int j = 1; j < length; j++) {
            int left = parent.get(j - 1);
            int right = j >= parent.size() ? 0 : parent.get(j);
            current.add(left + right);
        }
        result.add(current);
    }

    return result;
}