左右两边子数组的和相等

前缀和

class Solution {
    public int pivotIndex(int[] nums) {
        int a[] = new int[nums.length+1];
        int b[] = new int[nums.length+1];

        for(int i=1; i<= nums.length; i++) {
            a[i] = a[i-1] + nums[i-1]; 
        }
        for(int i=nums.length-1; i>=0; i--) {
            b[i] = b[i+1] + nums[i]; 
        }
        for(int i=0; i< nums.length; i++) {
            if(a[i] == b[i+1]) {
                return i;
            }
        }
        return -1;
    }
}