LeetCode 120. Triangle

问题描述

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.

问题分析

最开始的使用想使用贪心算法，每一步使用和最好的结果，发现不对。这个题目和给定一个二维数组，求起点到终点之间的子集的最小和。把上一层中每一元素组成的最小的子集记录下来，不断的迭代。

代码实现

public int minimumTotal(List<List<Integer>> triangle) {
if (triangle == null || triangle.size() == 0) {
return 0;
}
int[] result = new int[triangle.size()];
for (int i = 0; i < triangle.size(); i++) {
for (int j = triangle.get(i).size() - 1; j >= 0; j--) {
if (i == 0 || j == 0) {
result[j] = result[0] + triangle.get(i).get(j);
} else if (j == triangle.get(i).size() - 1) {
result[j] = result[j - 1] + triangle.get(i).get(j);
} else {
result[j] = triangle.get(i).get(j) + Math.min(result[j], result[j - 1]);
}
}
}
int min = Integer.MAX_VALUE;
for (int num : result) {
if (num < min) {
min = num;
}
}
return min;
}