724.find-pivot-index.py

#
# @lc app=leetcode id=724 lang=python3
#
# [724] Find Pivot Index
#
# https://leetcode.com/problems/find-pivot-index/description/
#
# algorithms
# Easy (54.54%)
# Likes:    6561
# Dislikes: 702
# Total Accepted:    777.7K
# Total Submissions: 1.4M
# Testcase Example:  '[1,7,3,6,5,6]'
#
# 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 <= 10^4
# -1000 <= nums[i] <= 1000
# 
# 
# 
# Note: This question is the same as 1991:
# https://leetcode.com/problems/find-the-middle-index-in-array/
# 
#

# @lc code=start
class Solution:
    def pivotIndex(self, nums):
        l, r = 0, sum(nums)
        for i in range(len(nums)):
            r -= nums[i]
            if l == r:
                return i
            l += nums[i]
        return -1

        
# @lc code=end