How do you solve Find Pivot Index on LeetCode?

Find Pivot Index (LeetCode #724) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(1) space complexity. Key insight: Using a total sum allows us to avoid nested loops, significantly improving efficiency.

What is the brute force solution for Find Pivot Index?

The brute force approach for Find Pivot Index is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach checks each index to see if the sum of elements to its left equals the sum of elements to its right. This is straightforward but inefficient as it requires recalculating sums repeatedly. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Find Pivot Index?

Find Pivot Index uses the following concepts: Array, Prefix Sum. The recommended patterns to study are: Prefix Sum, Two Pointers.

What are the interview tips for Find Pivot Index?

Always consider edge cases, such as arrays with only one element. Explain your thought process clearly while coding; it shows your understanding. If you get stuck, try to simplify the problem or break it down into smaller parts.

What are common mistakes when solving Find Pivot Index?

Forgetting to account for the current element when calculating the right sum. Not initializing the left sum correctly or updating it in each iteration.

#724

Find Pivot Index

Easy

ArrayPrefix SumPrefix SumTwo Pointers

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(1)

💡

Intuition

Time O(n)Space O(1)

The optimal approach uses a single pass to calculate the total sum of the array and then iteratively checks for the pivot index by maintaining a running left sum. This avoids redundant calculations and reduces time complexity.

⚙️

Algorithm

6 steps

1Step 1: Calculate the total sum of the array.
2Step 2: Initialize a variable 'leftSum' to 0.
3Step 3: Loop through each index 'i' in the array.
4Step 4: Calculate the right sum as total sum minus left sum minus the current element.
5Step 5: If left sum equals right sum, return the current index 'i'.
6Step 6: Update left sum by adding the current element and continue.

solution.py11 lines

1# Full working Python code
2
3def pivotIndex(nums):
4    total_sum = sum(nums)
5    left_sum = 0
6    for i in range(len(nums)):
7        right_sum = total_sum - left_sum - nums[i]
8        if left_sum == right_sum:
9            return i
10        left_sum += nums[i]
11    return -1

ℹ

Complexity note: This complexity is achieved because we only traverse the array twice: once to calculate the total sum and once to find the pivot index.

1Using a total sum allows us to avoid nested loops, significantly improving efficiency.
2Maintaining a running left sum helps us dynamically calculate the right sum without additional loops.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.