Paint House II

There are a row of n houses, each house can be painted with one of the k colors. 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 k cost matrix. For example, costs0 is the cost of painting house 0 with color 0; costs1 is the cost of painting house 1 with color 2, and so on... Find the minimum cost to paint all houses.

Note: All costs are positive integers.

Follow up: Could you solve it in O(nk) runtime?

 

public class Solution {
    public int minCostII(int[][] costs) {
        if(costs != null && costs.length == 0) return 0;
        int prevMin = 0, prevSec = 0, prevIdx = -1;
        for(int i = 0; i < costs.length; i++){
            int currMin = Integer.MAX_VALUE, currSec = Integer.MAX_VALUE, currIdx = -1;
            for(int j = 0; j < costs[0].length; j++){
                costs[i][j] = costs[i][j] + (prevIdx == j ? prevSec : prevMin);
                // 找出最小和次小的，最小的要记录下标，方便下一轮判断
                if(costs[i][j] < currMin){
                    currSec = currMin;
                    currMin = costs[i][j];
                    currIdx = j;
                } else if (costs[i][j] < currSec){
                    currSec = costs[i][j];
                }
            }
            prevMin = currMin;
            prevSec = currSec;
            prevIdx = currIdx;
        }
        return prevMin;
    }
}