算法设计Week15 LeetCode Algorithms Problem #120 Triangle
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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).
解法分析:
本题依旧可以使用动态规划算法,但和一般的动态规划算法不同的是,可以将问题改为自下而上的想法进行解决。首先,我们使用sum[j]
来表示某一行第i个元素从下向上所需的最小值。很快,我们就可以得到递推式:sum[j] = min(sum[j],sum[j + 1]) + triangle[i][j]
。其中i表示第i行。根据递推式,在遍历完整个triangle后,sum[0]
就是我们要的值。使用这种方法会比从下向上计算方便很多。算法的空间复杂度为
解法代码:
class Solution {public: int minimumTotal(vector<vector<int>>& triangle) { int row = triangle.size(); if(row == 0) return 0; vector<int> sum(triangle.back()); for(int i = row - 2; i >= 0; i--){ for(int j = 0; j <= i; j++){ sum[j] = min(sum[j], sum[j + 1]) + triangle[i][j]; } } return sum[0]; }};
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