LeetCode 364. Nested List Weight Sum II

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：

The question is asking for weight sum. The top level has largest depth.

Thus the top level need to be added maximum depth times.

We could accumlate the sum of Integer of the current level, add it to the res.

For next level, we could continue adding Integer to the same sum, add it to the res. Then upper level sum is added multiple times.

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:

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *     // Constructor initializes an empty nested list.
 *     public NestedInteger();
 *
 *     // Constructor initializes a single integer.
 *     public NestedInteger(int value);
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // Set this NestedInteger to hold a single integer.
 *     public void setInteger(int value);
 *
 *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 *     public void add(NestedInteger ni);
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
class Solution {
    public int depthSumInverse(List<NestedInteger> nestedList) {
        int weight = 0;
        int unweight = 0;
        while(!nestedList.isEmpty()){
            List<NestedInteger> nextList = new ArrayList<NestedInteger>();
            for(NestedInteger item : nestedList){
                if(item.isInteger()){
                    unweight += item.getInteger();
                }else{
                    nextList.addAll(item.getList());
                }
            }
            weight += unweight;
            nestedList = nextList;
        }
        return weight;
    }
}

Could also do DFS.

DFS return value needs both maximum depth and accumlated sum.

It does DFS bottom-up.

Time Complexity: O(n+m).

Space: O(n).

AC Java:

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *     // Constructor initializes an empty nested list.
 *     public NestedInteger();
 *
 *     // Constructor initializes a single integer.
 *     public NestedInteger(int value);
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // Set this NestedInteger to hold a single integer.
 *     public void setInteger(int value);
 *
 *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 *     public void add(NestedInteger ni);
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
class Solution {    
    public int depthSumInverse(List<NestedInteger> nestedList) {
        if(nestedList == null || nestedList.size() == 0){
            return 0;
        }
        
        int [] res = dfs(nestedList);
        return res[1];
    }
    
    private int [] dfs(List<NestedInteger> nestedList){
        if(nestedList == null || nestedList.size() == 0){
            return new int[]{0, 0};
        }
        
        int levelSum = 0;
        List<NestedInteger> nexts = new ArrayList<>();
        for(NestedInteger ni : nestedList){
            if(ni.isInteger()){
                levelSum += ni.getInteger();
            }else{
                nexts.addAll(ni.getList());
            }
        }
        
        int [] arr = dfs(nexts);
        return new int[]{arr[0]+1, levelSum*(arr[0]+1)+arr[1]};
    }
}

类似Nested List Weight Sum.