1020-partition-array-into-three-parts-with-equal-sum.js

/**
 * @param {number[]} A
 * @return {boolean}
 */
const canThreePartsEqualSum = function(A) {
  let lo = 0
  let hi = A.length - 1
  let lsum = 0
  let hsum = 0
  const sum = A.reduce((ac, el) => ac + el, 0)
  if(sum % 3 !== 0) return false
  const target = sum / 3

  while(lo < hi && lsum !== target) {
    lsum += A[lo]
    lo++
  }
  if(lsum !== target) return false
  while(lo < hi && hsum !== target) {
    hsum += A[hi]
    hi--
  }
  if(hsum !== target) return false

  let msum = 0
  for(let i = lo; i <= hi; i++) {
    msum += A[i]
  }
  if(msum !== target) return false
  return true
};