find pivot index

Author: always-maap

724.FindPivotIndex.py

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+"""
+724. Find Pivot Index
+Easy
+Array | Prefix Sum
+---
+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
+
+Note: This question is the same as 1991: https://leetcode.com/problems/find-the-middle-index-in-array/
+"""
+
+from typing import List
+
+
+# O(n) time | O(1) space
+class Solution:
+    def pivotIndex(self, nums: List[int]) -> int:
+        left, right = 0, sum(nums)
+        for (idx, num) in enumerate(nums):
+            right -= num
+            if left == right:
+                return idx
+            left += num
+        return -1