LeetCode Triangle Java版本

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题目描述：

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.

题目分析：

我的思路是这样的：先用贪婪算法求出一个局部最优解，然后用深度优先算法去遍历，不断替换最优解。结果超时了。。。

代码如下：

public class Solution {    int minSum = 0;    int tempSum = 0;    int rows = 0;    List<List<Integer>> tri;        public void deepSearch(int rowNum, int index){        List<Integer> rowList = tri.get(rowNum);        tempSum += rowList.get(index);        if(rowNum == rows-1){            if(tempSum < minSum)                minSum = tempSum;            tempSum = tempSum - rowList.get(index);            return;        }        deepSearch(rowNum+1,index);        deepSearch(rowNum+1,index+1);    }        public int minimumTotal(List<List<Integer>> triangle) {        rows = triangle.size();        if(rows == 0)            return 0;                int index = 0;        List<Integer> rowList = triangle.get(0);        minSum += rowList.get(0);        for(int i=1;i<rows;i++){            rowList = triangle.get(i);            if(rowList.get(index) <= rowList.get(index+1)){                minSum += rowList.get(index);            }            else{                minSum += rowList.get(index+1);                index = index+1;            }        }                tri = triangle;        deepSearch(0,0);        return minSum;    }}

这个代码自我感觉写的很糟糕。写起来比较费劲，而且不是最优的解法，而且还超时了。自己太过偷懒，什么问题都想着用递归去解决。

看了别人的解法，用的DP，十分简洁，让我很想记录下来。代码如下：

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

很佩服这样的代码，自己还是差很多啊。。

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