339_Nested List Weight Sum

339. Nested List Weight Sum

Level: easy

Tag: depth-first search

Question

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 1: BFS, recursive

flatten a list of lists

Time: $O(n)$

Space: $O(1)$

# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
#class NestedInteger(object):
#    def __init__(self, value=None):
#        """
#        If value is not specified, initializes an empty list.
#        Otherwise initializes a single integer equal to value.
#        """
#
#    def isInteger(self):
#        """
#        @return True if this NestedInteger holds a single integer, rather than a nested list.
#        :rtype bool
#        """
#
#    def add(self, elem):
#        """
#        Set this NestedInteger to hold a nested list and adds a nested integer elem to it.
#        :rtype void
#        """
#
#    def setInteger(self, value):
#        """
#        Set this NestedInteger to hold a single integer equal to value.
#        :rtype void
#        """
#
#    def getInteger(self):
#        """
#        @return the single integer that this NestedInteger holds, if it holds a single integer
#        Return None if this NestedInteger holds a nested list
#        :rtype int
#        """
#
#    def getList(self):
#        """
#        @return the nested list that this NestedInteger holds, if it holds a nested list
#        Return None if this NestedInteger holds a single integer
#        :rtype List[NestedInteger]
#        """

class Solution(object):
    def depthSum(self, nestedList):
        """
        :type nestedList: List[NestedInteger]
        :rtype: int
        """
        return self.depthPartialSum(nestedList, 0)

    def depthPartialSum(self, List, depth):
        sum = 0
        for i in List:
            if i.isInteger():
                sum += i.getInteger() * (depth + 1)
            else:
                sum += self.depthPartialSum(i.getList(), depth + 1)
        return sum

Solution 2: BFS, iterative

Time: $O(n)$

Space: $O(n)$

class Solution(object):
    def depthSum(self, nestedList):
        """
        :type nestedList: List[NestedInteger]
        :rtype: int
        """
        depth, ans = 1, 0
        while nestedList:
            ans += depth * sum([x.getInteger() for x in nestedList if x.isInteger()])
            nestedList = sum([x.getList() for x in nestedList if not x.isInteger()], [])
            depth += 1
        return ans

Solution 3: DFS, using stack

class Solution(object):
    def depthSum(self, nestedList):
        """
        :type nestedList: List[NestedInteger]
        :rtype: int
        """
        if len(nestedList) == 0: 
            return 0

        stack = []
        sum = 0
        for n in nestedList:
            stack.append((n, 1))
        while stack:
            next, d = stack.pop(0)
            if next.isInteger():
               sum += d * next.getInteger()
            else:
                for i in next.getList():
                    stack.append((i,d+1))
        return sum