/求一个整形数组中一个index，index左边的和 等于index 右边的和/
public int pivotIndex(int[] nums) {
long sum=0;
for(int s:nums){ sum +=s;}
long crunSum=0;
for(int i =0; i<nums.length; i++){
if( (sum - crunSum-nums[i]) == crunSum ){ return i;}
crunSum += nums[i];
}
return -1;
}