class Solution {

public int pivotIndex(int[] nums) {

if(nums == null || nums.length <= 1) return -1;

int n = nums.length;

int[] ps = new int[n+1];

ps[n] = 0;

for(int i = n-1; i >= 0; i--) {

ps[i] = ps[i+1] + nums[i];

}

int sum = 0;

for(int i = 0; i < n; i++) {

if(sum == ps[i+1]) return i;

sum += nums[i];

}

return -1;

}

}

// much easier to write code

class Solution {

public int pivotIndex(int[] nums) {

if(nums == null || nums.length == 0) throw new IllegalArgumentException("x");

int sum = 0;

int n = nums.length;

for(int i = 0; i < n; i++) {

sum += nums[i];

}

int ls = 0;

for(int i = 0; i<n; i++) {

if(sum-ls-nums[i] == ls) return i;

ls += nums[i];

}

return -1;

}

}