leetcode: triangle
阿新 • • 發佈:2019-07-27
題目描述:
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 is11(i.e., 2 + 3 + 5 + 1 = 11).
解題思路:
- 這道題顯然符合多階段決策模型,故可以考慮用動態規劃的解題思想去解題
- 注意題目中的資訊,每一行可以選擇下一行相鄰的兩個元素,而不是每一行中最小的元素
程式碼如下:
public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
// 用 sum 來記載每個階段的變化值
int [] sum = new int[triangle.size()];
ArrayList last = triangle.get(triangle.size() - 1);
for(int i = 0; i < last.size(); i++){
sum[i] = (int)last.get(i);
}
for(int i = triangle.size() - 2; i >= 0; i--){
for(int j = 0; j < triangle.get(i).size(); j++) {
// 狀態轉移方程
sum[j] = triangle.get(i).get(j) + Math.min(sum[j],sum[j + 1]);
}
}
return su