思路
有关子数组和的问题可以思考能否用前缀和方法解决

class Solution {
public:
    int pivotIndex(vector<int>& nums) {
        int sum = 0;
        for (int& n : nums) {
            sum += n;
        }
        int preSum = 0;
        for (int i = 0; i < nums.size(); ++i) {
            if (2 * preSum + nums[i] == sum) return i;
            else preSum += nums[i];
            }
        return -1;
    }
};