256. Paint House

题目：

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.

链接： http://leetcode.com/problems/paint-house/

题解：

房子用RGB刷漆， 每种漆对于每栋房子来说花费不一样，要求相邻两房子不一个色，并且花费最小。 这道题看提示需要用dp做。一开始我想设一个sum，一个last color = -1，不过没办法解决duplicate的问题。所以还是参考了jianchao.li大神的代码，对RGB分别进行dp。

Time Complexity - O(n)， Space Complexity - O(1)

public class Solution {
    public int minCost(int[][] costs) {  //dp
        if(costs == null || costs.length == 0) {
            return 0;
        }
        int len = costs.length, red = 0, blue = 0, green = 0;
        for(int i = 0; i < costs.length; i++) {
            int prevRed = red, prevBlue = blue, prevGreen = green;
            red = costs[i][0] + Math.min(prevBlue, prevGreen);
            blue = costs[i][1] + Math.min(prevRed, prevGreen);
            green = costs[i][2] + Math.min(prevRed, prevBlue);
        }
        
        return Math.min(red, Math.min(blue, green));
    }
}

二刷:

继续锻炼思维能力，现在写这类题目已经比较轻松了，看来自己真的是在进步的。

Java:

Time Complexity - O(n)， Space Complexity - O(1)

public class Solution {
    public int minCost(int[][] costs) {
        if (costs == null || costs.length == 0) return 0;
        int pRed = 0, pGreen = 0, pBlue = 0;
        for (int[] cost : costs) {
            int lastRed = pRed, lastGreen = pGreen, lastBlue = pBlue;
            pRed = cost[0] + Math.min(lastGreen, lastBlue);
            pGreen = cost[1] + Math.min(lastRed, lastBlue);
            pBlue = cost[2] + Math.min(lastRed, lastGreen);
        }
        return Math.min(pRed, Math.min(pGreen, pBlue));
    }
}

Reference:

https://leetcode.com/discuss/51721/simple-java-dp-solution