[LeetCode] 120. Triangle

Problem

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-top DP

class Solution {
    public int minimumTotal(List<List<Integer>> triangle) {
        int[] dp = new int[triangle.size()+1];
        for (int i = triangle.size()-1; i >= 0; i--) {
            for (int j = 0; j <= i; j++) {
                dp[j] = Math.min(dp[j], dp[j+1]) + triangle.get(i).get(j);
            }
        }
        return dp[0];
    }
}

Non Extra Space DP

class Solution {
    public int minimumTotal(List<List<Integer>> triangle) {
        int len = triangle.size();
        for (int i = len-2; i >= 0; i--) {
            for (int j = 0; j <= i; j++) {
                int preMin = Math.min(triangle.get(i+1).get(j), triangle.get(i+1).get(j+1));
                int curMin = preMin + triangle.get(i).get(j);
                triangle.get(i).set(j, curMin);
            }
        }
        return triangle.get(0).get(0);
    }
}