leetcode — triangle

/**
 * Source : https://oj.leetcode.com/problems/triangle/
 *
 *
 * Given a triangle, find the minimum path sum from top to bottom.
 * Each step you may move to adjacent numbers on the row below.
 *
 * For example, given the following triangle
 *
 * [
 *      [2],
 *     [3,4],
 *    [6,5,7],
 *   [4,1,8,3]
 * ]
 *
 * The minimum path sum from top to bottom is 11 (i.e., 2 + 3 + 5 + 1 = 11).
 *
 * Note:
 * Bonus point if you are able to do this using only O(n) extra space, where n is the total number of rows in the triangle.
 *
 */
public class Triangle {

    /**
     * 求出三角形中和最小的路径
     * 使用常数空间O(n)
     * 从最后一层开始计算，第i层的个数为i个，每个元素和第i+1层的左右下角两个元素中较小的一个进行求和作为该位置新的元素
     *
     *
     * @param triangle
     * @return
     */
    public int getMinimumPath (int[][] triangle) {
        if (triangle.length == 0) {
            return 0;
        }
        int m = triangle.length;
        int n = triangle[0].length;
        int[] result = triangle[triangle.length-1];
        for (int i = m-2; i >= 0; i--) {
            for (int j = 0; j <= i; j++) {
                result[j] = Math.min(result[j], result[j+1]) + triangle[i][j];
            }
        }
        return result[0];
    }

    public static void main(String[] args) {
        Triangle triangle = new Triangle();
        int[][] arr = new int[][]{
                {2},
                {3,4},
                {6,5,7},
                {4,1,8,3}
        };

        System.out.println(triangle.getMinimumPath(arr));
    }
}