Description
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 + 42 + 63 = 27)

DFS solution

Complexity Analysis

The algorithm takes O(N)O(N) time, where NN is the total number of nested elements in the input list. For example, the list [ [[[[1]]]], 2 ] contains 44 nested lists and 22 nested integers (11 and 22), so N=6N=6.

In terms of space, at most O(D)O(D) recursive calls are placed on the stack, where DD is the maximum level of nesting in the input. For example, D=2D=2 for the input [[1,1],2,[1,1]], and D=3D=3 for the input [1,[4,[6]]].

/**
 * // 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) {
        return dfs(nestedList, 1);
    }
    
    private int dfs(List<NestedInteger> nestedList, int level) {
        if (nestedList == null) return 0;
        
        int sum = 0;
        for (NestedInteger ni : nestedList) {
            if (ni.isInteger()) {
                sum += ni.getInteger() * level;
            } else {
                sum += dfs(ni.getList(), level+1);
            }
        }
        return sum;
        
    }
}

imgRuntime: 2ms