Python: Find the indices of three numbers that sum to 0 in a list

Python Programming Puzzles: Exercise-72 with Solution

Indices of Three Numbers Summing to Zero

Write a Python program to find the indices of three numbers that sum to 0 in a given list of numbers.

Input: [12, -7, 3, -89, 14, 4, -78, -1, 2, 7]
Output:
[1, 2, 5]

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

Visual Presentation:

Sample Solution:

Python Code:

# License: https://bit.ly/3oLErEI

# Define a function named 'test' that takes a list of numbers as input
def test(nums):
    inv = {n: i for i, n in enumerate(nums)}  # Create a dictionary to store the indices of numbers
    # Note that later duplicates will override earlier entries
    
    # Iterate through the indices and numbers in 'nums'
    for i, n in enumerate(nums):
        if inv[n] == i:
            del inv[n]  # Remove the entry if it corresponds to the current index
        
        # Check if there is a pair of indices (j, k) such that nums[j] + nums[k] + n = 0
        if any((-m - n) in inv for m in nums[:i]):
            # If a solution is found, get the indices (j, m) where nums[j] + m = -n
            j, m = next((j, m) for j, m in enumerate(nums) if (-m - n) in inv)
            # Get the index k for nums[k] = -m - n
            k = inv[-m - n]
            return sorted([i, j, k])  # Return the sorted list of indices

# Example 1
nums1 = [12, -7, 3, -89, 14, 4, -78, -1, 2, 7]
print("List of numbers:", nums1)
print("Indices of three numbers that sum to 0 in the said list:")
print(test(nums1))

# Example 2
nums2 = [1, 2, 3, 4, 5, 6, -7]
print("\nList of numbers:", nums2)
print("Indices of three numbers that sum to 0 in the said list:")
print(test(nums2))

Sample Output:

List of numbers: [12, -7, 3, -89, 14, 4, -78, -1, 2, 7]
Indices of three numbers that sum to 0 in the said list:
[1, 2, 5]

List of numbers: [1, 2, 3, 4, 5, 6, -7]
Indices of three numbers that sum to 0 in the said list:
[2, 3, 6]

Flowchart:

Python Code Editor :

Have another way to solve this solution? Contribute your code (and comments) through Disqus.

Previous: Inject a number in between each pair of adjacent numbers in a list of numbers.
Next: Find a string contains a vowel between two consonants, in a given string.