LeetCode – Triangle-优快云博客

转自：http://www.programcreek.com/2013/01/leetcode-triangle-java/

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.

Bottom-Up (Good Solution)

We can actually start from the bottom of the triangle.

public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
	int[] total = new int[triangle.size()];
	int l = triangle.size() - 1;
 
	for (int i = 0; i < triangle.get(l).size(); i++) {
		total[i] = triangle.get(l).get(i);
	}
 
	// iterate from last second row
	for (int i = triangle.size() - 2; i >= 0; i--) {
		for (int j = 0; j < triangle.get(i + 1).size() - 1; j++) {
			total[j] = triangle.get(i).get(j) + Math.min(total[j], total[j + 1]);
		}
	}
 
	return total[0];
}