Paint House I -- DP -- Leetcode

Question:

There are a row of n houses, each house can be painted with one of the three colors: red, blue or green. The cost of painting each house with a certain color is different. You have to paint all the houses such that no two adjacent houses have the same color.

The cost of painting each house with a certain color is represented by a n x 3 cost matrix. For example, costs[0][0] is the cost of painting house 0 with color red;costs[1][2] is the cost of painting house 1 with color green, and so on... Find the minimum cost to paint all houses.

Note:
All costs are positive integers.

Answer:
Time: O(n * 3) = O(n), Space: O(1)
public int minCost(int[][] costs) {
        if(costs==null)return 0;
        int m = costs.length;
        if(m==0)return 0;
     
        for(int i=1;i<m;++i){
            costs[i][0] += Math.min(costs[i-1][1], costs[i-1][2]);
            costs[i][1] += Math.min(costs[i-1][0], costs[i-1][2]);
            costs[i][2] += Math.min(costs[i-1][0], costs[i-1][1]);
        }
        int res = Math.min(costs[m-1][0], Math.min(costs[m-1][1], costs[m-1][2]));
        return res;
    }