Partition Array Into Three Parts With Equal Sum

Description

Given an array A of integers, return true if and only if we can partition the array into three non-empty parts with equal sums.

Formally, we can partition the array if we can find indexes i+1 < j with (A[0] + A[1] + ... + A[i] == A[i+1] + A[i+2] + ... + A[j-1] == A[j] + A[j-1] + ... + A[A.length - 1])

 

Example 1:

Input: A = [0,2,1,-6,6,-7,9,1,2,0,1]
Output: true
Explanation: 0 + 2 + 1 = -6 + 6 - 7 + 9 + 1 = 2 + 0 + 1

Example 2:

Input: A = [0,2,1,-6,6,7,9,-1,2,0,1]
Output: false

Example 3:

Input: A = [3,3,6,5,-2,2,5,1,-9,4]
Output: true
Explanation: 3 + 3 = 6 = 5 - 2 + 2 + 5 + 1 - 9 + 4

 

Constraints:

• 3 <= A.length <= 50000
• -10^4 <= A[i] <= 10^4

Solution(javascript)

/** 这个没看清题，题目的意思是按顺序相加的，那就简单了
 * @param {number[]} A
 * @return {boolean}
 */
// const canThreePartsEqualSum = function (A) {
//   const map = {}
//   const sum = A.reduce((acc, v) => {
//     map[v] = (map[v] || 0) + 1
//     return acc + v
//   }, 0)
//   if (!Number.isInteger(sum / 3)) {
//     return false
//   }
//   let count = 0
//   let currentSum = 0
//   const target = sum / 3
//   for (const num of A) {
//     if (map[num] > 0) {
//       map[num] -= 1
//       currentSum += num
//       if (currentSum === target) {
//         count += 1
//         currentSum = 0
//       } else if (map[target - currentSum] > 0) {
//         map[target - currentSum] -= 1
//         currentSum = 0
//         count += 1
//       }
//     }
//   }
//   return count === 3
// }

const canThreePartsEqualSum = function (A) {
  const sum = A.reduce((acc, v) => acc + v, 0)
  if (!Number.isInteger(sum / 3)) {
    return false
  }
  let count = 0
  let currentSum = 0
  const target = sum / 3
  for (const num of A) {
    currentSum += num
    if (currentSum === target) {
      currentSum = 0
      count += 1
    }
  }
  return count === 3 && currentSum === 0
}