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)

Solution1：BFS

Time Complexity: O(N) Space Complexity: O(N)

Solution2：DFS(recursive)

Time Complexity: O(N) Space Complexity: O(N)

Solution3：DFS(stack)

Time Complexity: O(N) Space Complexity: O(N)

Solution1 Code：

public class Solution {
    public int depthSum(List<NestedInteger> nestedList) {
        if(nestedList == null){
            return 0;
        }

        int sum = 0;
        int level = 1;

        Queue<NestedInteger> queue = new LinkedList<NestedInteger>(nestedList);
        while(queue.size() > 0){
            int size = queue.size();

            for(int i = 0; i < size; i++){
                NestedInteger ni = queue.poll();

                if(ni.isInteger()){
                    sum += ni.getInteger() * level;
                } else {
                    queue.addAll(ni.getList());
                }
            }
            level++;
        }
        return sum;
    }
}

Solution2 Code：

class Solution {
    public int depthSum(List<NestedInteger> nestedList) {
        return depthSum(nestedList, 1);
    }
    public int depthSum(List<NestedInteger> nestedList, int level) {
        int result = 0;
        for(NestedInteger ni : nestedList) {
            if (ni.isInteger()) {
                result += (level * ni.getInteger());
            }else {
                result += depthSum(ni.getList(), level + 1);
            }
        }
        return result;
    }
}

Solution3 Code：

public class Solution {
    public int depthSum(List<NestedInteger> nestedList) {
        int res = 0;
        Stack<Iterator<NestedInteger>> stk = new Stack<>();
        stk.push (nestedList.iterator());
        
        while (!stk.isEmpty()) {
            Iterator<NestedInteger> itr = stk.peek();
            while (itr.hasNext()) {
                NestedInteger n = itr.next();
                if (n.isInteger()) res += n.getInteger() * stk.size ();
                else {
                    stk.push(n.getList().iterator());
                    break;
                }
            }
            if (!stk.peek().hasNext()) stk.pop();
        }
        return res;
    }
}