Partition Array Into Three Parts With Equal Sum

求一个数组能不能分为三个小数组，并且这三个小数组的和相等。我们已知目标是三个小数组，所以数组能被3整除。然后再找到小数组目标和是多少。计算是否有三个小数组的目标和。

class Solution {
    public boolean canThreePartsEqualSum(int[] A) {
        int sum = 0;
        for(int a : A) {
            sum += a ; // calculate sum
        }
        if(sum % 3 != 0) {
            return false; // divide by 3 ?
        }
        sum /= 3; // get the sum goal
        int count = 0; // count how many sum we have 
        int tmp = 0;
        for(int a : A) {
            tmp += a;
            if(tmp == sum){ // if we find one
                count++;
                tmp = 0;
            }
            if(count == 3) // if there 3
                return true;
        }
        return false;
    }
}