leetcode — pascals-triangle-ii

import java.util.Arrays;

/**
 *
 * Source : https://oj.leetcode.com/problems/pascals-triangle-ii/
 *
 *
 * Given an index k, return the kth row of the Pascal's triangle.
 *
 * For example, given k = 3,
 * Return [1,3,3,1].
 *
 * Note:
 * Could you optimize your algorithm to use only O(k) extra space?
 *
 */
public class PascalTriangle2 {

    /**
     * 获取杨辉三角的第n行
     * 占用常数空间，使用长度为n的数组，依次计算 ...i-2,i-2,i 行的数据，可以根据俄第i-1行的结果来计算i行，
     * 计算每行数据的时候，如果从前向后填入的话，会把后面的覆盖就不能继续计算，那么就从后向前
     *
     * @param n
     * @return
     */
    public int[] getRow (int n) {
        int[] row = new int[n];
        row[0] = 1;
        if (n == 1) {
            return row;
        }
        for (int i = 2; i <= n; i++) {
            for (int j = i; j > 1; j--) {
                row[j-1] = row[j-1] + row[j-2];
            }
        }
        return row;
    }

    public static void main(String[] args) {
        PascalTriangle2 pascalTriangle2 = new PascalTriangle2();
        System.out.println(Arrays.toString(pascalTriangle2.getRow(5)));
    }
}