724_Find Pivot Index

Given an array of integers nums, write a method that returns the "pivot" index of this array.

We define the pivot index as the index where the sum of the numbers to the left of the index is equal to the sum of the numbers to the right of the index.

If no such index exists, we should return -1. If there are multiple pivot indexes, you should return the left-most pivot index.

Example 1:

Input: nums = [1, 7, 3, 6, 5, 6]
Output: 3
Explanation: The sum of the numbers to the left of index 3 (nums[3] = 6) is equal to the sum of numbers to the right of index 3.
Also, 3 is the first index where this occurs.

Example 2:

Input: nums = [1, 2, 3]
Output: -1
Explanation: There is no index that satisfies the conditions in the problem statement.

Solution:

Idea:

• First compute the sum of all elements in the array as s.

• Loop over each element, add sequentially to get the leftSum, and check if the leftSum equals to the rightSum which equals total sum - current element - leftSum.

Time Complexity: $O(n)$

Space Complexity: $O(1)$

def pivotIndex(nums):
    """
    :type nums: List[int]
    :rtype: int
    """
    s = sum(nums)
    leftSum = 0 # sum of elements before current element
    for i, x in enumerate(nums):
        if leftSum * 2 == s - x:
            return i
        leftSum += x
    return -1