724. Find Pivot Index

• python3

Problem

Given an array of integers nums, calculate the pivot index of this array.

The pivot index is the index where the sum of all the numbers strictly to the left of the index is equal to the sum of all the numbers strictly to the index's right.

If the index is on the left edge of the array, then the left sum is 0 because there are no elements to the left. This also applies to the right edge of the array.

Return the leftmost pivot index. If no such index exists, return -1.

Example 1:

Input: nums = [1,7,3,6,5,6]
Output: 3
Explanation:
The pivot index is 3.
Left sum = nums[0] + nums[1] + nums[2] = 1 + 7 + 3 = 11
Right sum = nums[4] + nums[5] = 5 + 6 = 11

Example 2:

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

Example 3:

Input: nums = [2,1,-1]
Output: 0
Explanation:
The pivot index is 0.
Left sum = 0 (no elements to the left of index 0)
Right sum = nums[1] + nums[2] = 1 + -1 = 0

Constraints:

• 1 <= nums.length <= 104
• -1000 <= nums[i] <= 1000

Pseudocode

1. Create two additional arrays:one for the left sum and one for the right sum.
2. Traverse the original array to fill the left and right sum arrays.
3. Compare the left sum and right sum arrays to find an index where both sums are equal.
4. Return the index. If no matching index is found, return -1.

Code

class Solution:
    def pivotIndex(self, nums: List[int]) -> int:
        
        length = len(nums)
        leftSum = [0] * length
        rightSum = [0] * length
        

        for i in range(1, length):
            leftSum[i] = leftSum[i-1] +nums[i-1]
            rightSum[length-i-1] = rightSum[length-i] + nums[length-i]

        for i in range(0, length):
            if leftSum[i] == rightSum[i]:
                return i

        return -1

Result

• Time Complexity : O(n) because we traverse the array twice.
• Space Complextiy : O(n) because we use two additional arrays.

Review

• Actually, it is not my original solution. I accidentally saw the hints . T.T