Codewars|Equal Sides Of An Array

DESCRIPTION:

You are going to be given an array of integers. Your job is to take that array and find an index N where the sum of the integers to the left of N is equal to the sum of the integers to the right of N. If there is no index that would make this happen, return -1.

For example:

Let's say you are given the array {1,2,3,4,3,2,1}:
Your function will return the index 3, because at the 3rd position of the array, the sum of left side of the index ({1,2,3}) and the sum of the right side of the index ({3,2,1}) both equal 6.

Let's look at another one.
You are given the array {1,100,50,-51,1,1}:
Your function will return the index 1, because at the 1st position of the array, the sum of left side of the index ({1}) and the sum of the right side of the index ({50,-51,1,1}) both equal 1.

Last one:
You are given the array {20,10,-80,10,10,15,35}
At index 0 the left side is {}
The right side is {10,-80,10,10,15,35}
They both are equal to 0 when added. (Empty arrays are equal to 0 in this problem)
Index 0 is the place where the left side and right side are equal.

Note: Please remember that in most programming/scripting languages the index of an array starts at 0.

Input:
An integer array of length 0 < arr < 1000. The numbers in the array can be any integer positive or negative.

Output:
The lowest index N where the side to the left of N is equal to the side to the right of N. If you do not find an index that fits these rules, then you will return -1.

Note:
If you are given an array with multiple answers, return the lowest correct index.

思路：

设置一个指针，指针左边求和，指针右边求和。当左右相等的时候，跳出循环，直接返回。

Solution：

function findEvenIndex(arr)
{
  let leftSum = 0; // 左边初始值
  let rightSum = arr.reduce((prev, cur) => prev + cur, 0); // 右边和的默认初始值
  
  // 指针从左往右，每往右移一个，rightSum就减少，leftSum就增加
  for (let i = 0; i < arr.length; i++) {
    rightSum -= arr[i];
    if (i > 0) leftSum += arr[i - 1];
    
    if (leftSum === rightSum) return i;
  }
  return -1;
}

优化之后就变成了：

function findEvenIndex(arr)
{
  const sum = arr => arr.reduce((acc, cur) => (acc + cur), 0);
  return arr.findIndex((val, index) => sum(arr.slice(0, index)) === sum(arr.slice(index + 1)))
  
}