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Python Program to Find the Total Sum of a Nested List Using Recursion

  • Difficulty Level : Medium
  • Last Updated : 25 Apr, 2022

A nested list is given. The task is to print the sum of this list using recursion. A nested list is a list whose elements can also be a list. 

Examples : 

Input: [1,2,[3]]
Output: 6

Input: [[4,5],[7,8,[20]],100]
Output: 144

Input: [[1,2,3],[4,[5,6]],7]
Output: 28

Recursion: In recursion, a function calls itself repeatedly. This technique is generally used when a problem can be divided into smaller subproblems of the same form.


Iterate through the list and whenever we find that an element of the list is also a list that means we have to do the same task of finding the sum with this element list (which can be nested) as well.  So we have found a subproblem and, we can call the same function to perform this task and just change the argument to this sublist. And when the element is not a list, then simply add its value to the global total variable.


# Python Program to find sum
# of nested list using Recursion
def sum_nestedlist( l ):
    # specify that global variable is
    # referred to here in this function
    total = 0
    for j in range(len(l)):
        if type(l[j]) == list :
            # call the same function if
            # the element is a list
            total+= sum_nestedlist(l[j])
            # if it's a single element
            # and not a list, add it to total
            total += l[j]  
    return total

Time Complexity: O(N), Where N is the total number of elements in the list.
Auxiliary Space: O(1)

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