0724-find-pivot-index.rb

# frozen_string_literal: true

# 724. Find Pivot Index
# https://leetcode.com/problems/find-pivot-index
# Easy

=begin
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
=end

# Runtime 109 ms
# Memory 212.6 MB
# @param {Integer[]} nums
# @return {Integer}
def pivot_index(nums)
  sum = nums.sum
  left_sum = 0

  nums.each_with_index do |num, index|
    return index if left_sum == sum - left_sum - num

    left_sum += num
  end

  -1
end

# **************** #
#       TEST       #
# **************** #

require "test/unit"
class Test_pivot_index < Test::Unit::TestCase
  def test_
    assert_equal 3, pivot_index([1, 7, 3, 6, 5, 6])
    assert_equal(-1, pivot_index([1, 2, 3]))
    assert_equal 0, pivot_index([2, 1, -1])
  end
end