leetcode364- Nested List Weight Sum II- medium

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)

算法：层级遍历的BFS。数据结构：queue + int partial sum + int sum

1.先全推入

2.层级取出，如果是Integer就直接放到partialSum里，如果还是nested就把这个东西里下一层的所有东西推入下一层queue

3.每层结束后，让sum += partialSum，这样就能做到最早推进去的(最外层的)能加层数次，最后推进去的(最里层的)就只加一次。（这个小技巧比较tricky要掌握，这样就不用开一个list记录下来每一层的数字最后再乘了）

/**
 * // 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 sum = 0;
        int partialSum = 0;
        Queue<NestedInteger> q = new LinkedList<>();
        
        for (NestedInteger ni : nestedList) {
            q.offer(ni);
        }
        while (!q.isEmpty()) {
            int size = q.size();
            for (int i = 0; i < size; i++) {
                NestedInteger ni = q.poll();
                if (ni.isInteger()) {
                    partialSum += ni.getInteger();
                } else {
                    for (NestedInteger nni : ni.getList()) {
                        q.offer(nni);
                    }
                }
            }
            sum = sum + partialSum;
        }
        return sum;
    }
}