leetcode: Triangle

最新推荐文章于 2019-07-27 09:27:00 发布

martin_liang

最新推荐文章于 2019-07-27 09:27:00 发布

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分类专栏： C++/C 算法 leetcode

本文链接：https://blog.youkuaiyun.com/martin_liang/article/details/8942419

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本文介绍了一种求解三角形最小路径和的问题，通过动态规划的方法从顶点到底部找到路径总和最小的路径。示例中给出了具体的实现过程，并提供了一个C++示例代码。

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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 triangl

以下算法并不算最优算法

#define MIN_NUM  10000

class Solution {
//用动态规划的思想，从下到上或者从上到下
int  min(int a, int b)
{
    if (a <= b)
        return a;
    return b;
}

public:
    int minimumTotal(vector<vector<int> > &triangle) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
        
        int vectorSize = triangle.size();
        
        if (vectorSize == 1)
            return triangle[0][0];
        
        vector<int> Level1;
        vector<int> Level2;
        
        vector<int> *pPreLevel = &Level1;
        vector<int> *pCurLevel = &Level2;
        
                
        pPreLevel->push_back(triangle[0][0]);
        
        for(int curLevel = 1; curLevel < vectorSize; curLevel++)
        {
            int curLevelSize = triangle[curLevel].size();
            
            for (int i = 0; i < curLevelSize; i++)
            {
                if (i == 0)
                {
                    pCurLevel->push_back((*pPreLevel)[0]+triangle[curLevel][i]);
                }
                else if (i == curLevelSize-1)
                {
                    pCurLevel->push_back((*pPreLevel)[i-1]+triangle[curLevel][i]);
                }
                else
                {
                    pCurLevel->push_back(min((*pPreLevel)[i-1], (*pPreLevel)[i])+triangle[curLevel][i]);
                }
            }
            vector<int> *pTemp;
            
            pTemp = pPreLevel;
            pPreLevel->clear();
            
            pPreLevel = pCurLevel;
            pCurLevel = pTemp;
        }
        
        int minSum = MIN_NUM;
        for(int idx = 0; idx < pPreLevel->size(); idx++)
        {
            if ((*pPreLevel)[idx] < minSum)
            {
                minSum = (*pPreLevel)[idx];
            }
        }
        
        return minSum;
    }
};