/**
 * @param {number[]} arr
 * @return {boolean}
 */
var canThreePartsEqualSum = function(arr) {
        var oneThird = sum / 3,
        p1 = 0,
        p2 = A.length - 1,
        sum1 = 0,
        sum2 = sum,
        sum3 = 0;

    while (p1<p2 && (sum1 !== oneThird || sum2 !== oneThird)) {
        if (sum1!==oneThird) {
            sum1 += A[p1]
            sum2 -= A[p1] //sliding window
            p1++
        }
        if (sum3!==oneThird) {
            sum3 += A[p2]
            sum2 -= A[p2]
            p2--
        }

    }

     return sum1 === sum2 && sum2 === sum3 // this line could have more efficient alternatives

};




cpp:
class Solution {
public:
    bool canThreePartsEqualSum(vector<int>& arr) {
        //prefix sum with two pointers
        for(int i=1;i<arr.size();i++)
        {
            arr[i]+=arr[i-1];
        }
        int total=arr.back();
        if(total%3!=0) return false;
        int sum=total/3;

        int i=0,j=arr.size()-2;
        while(i<j)
        {
            while(i<j && arr[i]!=sum) i++;
            while(j>i && total-arr[j]!=sum) j--;
            if(i<j && arr[j]-arr[i]==sum) return true;
            i++;
            j--;
        }
        return false;
    }
};