339. Nested List Weight Sum

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.

Example 1:
Given the list [[1,1],2,[1,1]], return 10. (four 1's at depth 2, one 2 at depth 1)

Example 2:
Given the list [1,[4,[6]]], return 27. (one 1 at depth 1, one 4 at depth 2, and one 6 at depth 3; 1 + 4*2 + 6*3 = 27)

Solution:

We traverse the nested list level by level.

In each level, we check if the element is integer.

If it is, we can multiply the depth and add to the sum.

Otherwise, we put all the elements in the nested list to a new list and set it to be the nested list we traverse in the next round.



Code:

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *
 *     // @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();
 *
 *     // @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();
 * }
 */
public class Solution {
    public int depthSum(List<NestedInteger> nestedList) {
        int sum = 0;
        int depth = 1;
        while (!nestedList.isEmpty()) {
            List<NestedInteger> nextList = new ArrayList<>();
            for (NestedInteger n : nestedList) {
                if (n.isInteger()) {
                    sum += depth * n.getInteger();
                }
                else {
                    nextList.addAll(n.getList());
                }
            }
            nestedList = nextList;
            depth++;
        }
        return sum;
    }
}