LeetCode Nested List Weight Sum II-优快云博客

本文针对LeetCode上的题目《Nested List Weight Sum II》进行了解析，该题要求计算嵌套整数列表中所有整数的加权和，权重由底层向顶层递增。文章提供了一种类似于广度优先搜索的解决方案，并详细解释了其实现思路及时间与空间复杂度。

原题链接在这里：https://leetcode.com/problems/nested-list-weight-sum-ii/description/

题目：

Given a nested list of integers, return the sum of all integers in the list weighted by their depth.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Different from the previous question where weight is increasing from root to leaf, now the weight is defined from bottom up. i.e., the leaf level integers have weight 1, and the root level integers have the largest weight.

Example 1:
Given the list [[1,1],2,[1,1]], return 8. (four 1's at depth 1, one 2 at depth 2)

Example 2:
Given the list [1,[4,[6]]], return 17. (one 1 at depth 3, one 4 at depth 2, and one 6 at depth 1; 1*3 + 4*2 + 6*1 = 17)

题解：

像BFS. unweight一直累计每层的数. 但每层结束，都会再加一遍unweight到weight中.

这样最上层的数就被加了好几遍.

Time Complexity: O(n+m). n 是flat后一共有多少个数字, 就像BFS的node数. m 是层数，就像BFS的edge数.

Space: O(n).

AC Java:

 1 /**
 2  * // This is the interface that allows for creating nested lists.
 3  * // You should not implement it, or speculate about its implementation
 4  * public interface NestedInteger {
 5  *     // Constructor initializes an empty nested list.
 6  *     public NestedInteger();
 7  *
 8  *     // Constructor initializes a single integer.
 9  *     public NestedInteger(int value);
10  *
11  *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
12  *     public boolean isInteger();
13  *
14  *     // @return the single integer that this NestedInteger holds, if it holds a single integer
15  *     // Return null if this NestedInteger holds a nested list
16  *     public Integer getInteger();
17  *
18  *     // Set this NestedInteger to hold a single integer.
19  *     public void setInteger(int value);
20  *
21  *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
22  *     public void add(NestedInteger ni);
23  *
24  *     // @return the nested list that this NestedInteger holds, if it holds a nested list
25  *     // Return null if this NestedInteger holds a single integer
26  *     public List<NestedInteger> getList();
27  * }
28  */
29 class Solution {
30     public int depthSumInverse(List<NestedInteger> nestedList) {
31         int weight = 0;
32         int unweight = 0;
33         while(!nestedList.isEmpty()){
34             List<NestedInteger> nextList = new ArrayList<NestedInteger>();
35             for(NestedInteger item : nestedList){
36                 if(item.isInteger()){
37                     unweight += item.getInteger();
38                 }else{
39                     nextList.addAll(item.getList());
40                 }
41             }
42             weight += unweight;
43             nestedList = nextList;
44         }
45         return weight;
46     }
47 }